TLDR: This is 100% a scam.

My brother is working in a union and has met a bunch of people and started making good money lately

Let me rephrase: "my brother is 'fresh meat' -- he is new to this community so he doesn't know who is trustworthy; he has more money than he knows what to do with; he's got a taste of making good money and now wants more". That's all gold to a con man; that's their dream mark.

Yesterday, a colleague of his needed a favor because his car broke down, so my brother spent the night driving him home and dealing with a few other details.

Con men love it when marks "do them a favour" because it makes the con man seem vulnerable rather than threatening, and builds trust. People like doing favours, and I'm sure your brother is a nice person, but this favour was no accident. The car is fine. The con man asked your brother for "help" because your brother is the new mark in town.

Think about it from the perspective of the con man. Suppose you have come up with a scam involving the mark believing that an object they are holding is crazy valuable. What's the scam? Doesn't matter; the con man knows what it is. What's the object? Doesn't matter, as long as it is plausibly valuable. The key point is, for the scam to work you have to get the MacGuffin in the mark's hands somehow. Which means they have to be at your home, and receptive to receiving a valuable object.

How are you going to do that?

You need to establish a plausible reason to give the mark the MacGuffin at your place, and "in return for a favour" is just such a plausible reason.

What are you going to do, just wait around for an organic opportunity for a favour? Of course not. You're going to manufacture such an opportunity at a time that is convenient for you. And "car trouble" can happen at any time, for any reason.

And then what happens? Your brother spends the evening driving the guy around and helping him with his errands. What does the con man now know? Your brother can afford to keep a car, so he's got money and is not some bus-riding cheapskate. Your brother has enough free time to drop everything and drive someone around all night. He's not going home to his wife and kids right away after work. Let's make some small talk and find out, does he have a girlfriend, and would he like some extra cash to buy her something nice? And so on.

Where do we end up? At the con man's home, which is convenient because that's where he stores the MacGuffin. Then we get to giving the MacGuffin to the mark, in return for the mark's generosity. Which is now plausible, because they've been such good friends for the last couple hours.

Now, I note that the reason for giving the MacGuffin to the mark must not be too plausible. It must be somewhat implausible! Why? Surely the con is better if the reasoning is more plausible, right?

Wrong.

Con men give you deliberately implausible stories because they are looking for marks who will believe implausible stories! They're not trying to con hardened skeptics!

Your brother believed this barely-plausible story that this guy he doesn't know wants to go into business with him. Your brother is new to the community, and has ready cash, and believes barely-plausible stories. Of course the con man wants to be his new best friend!

In return (or maybe not, unsure of the details), this guy offered to have my brother sell some baseball cards that the colleague had, some of which were supposedly extremely valuable.

"Supposedly" is the key word, and good for you for using it. They are worthless.

He then gave my brother a sleeve of the cards he had without any monetary exchange.

Con men do that to again, build trust. Normal people are not in the habit of giving valuable items to people they just met.

I took a look at the cards this morning and each card is in those plastic protective cases.

Those are a penny a dozen. That establishes nothing.

True story: I have all 23 issues of the Marvel Doctor Who Comics reprint series from 1984 in just such plastic protective cases. They are not worth a quarter million dollars.

My brother was telling me about how some of them were pretty valuable

They are not. Your brother is not an expert in this field.

asked him to help sell them, and in return he'd get a third of the cut.

The con man wishes your brother to commit frauds on his behalf, and is keeping 70% of the profits in exchange for 0% of the going to jail when caught.

Or, it's some other scam. Only the con man knows for sure, but it is 100% not legit, whatever it is.

He then showed me the specific most valuable card, a 1990 Jose Uribe, which was, according to him, one of the first error cards, and one of only 1000 others. He showed me how it was printed off-center, such that the padding on the right was about twice as much as on the left. This card was supposedly worth $250,000 and he said how his friend showed him listings for it on eBay in that price range.

It's the "his friend showed him" that is the key there. The "friend" is the one who entered the listings on Ebay. I could list a ham sandwich for a million bucks on Ebay. That does not establish the price of ham sandwiches at a million bucks.

Well, having read a lot on money.se and around the Internet about how scams work, I was immediately suspicious and told him some basic rules on how to avoid scams (don't give him money first, make sure checks don't bounce, etc) and he said that his friend was super cool, that he trusted him, but that he would be careful.

The "he trusted him" is the key part. It is already too late; the con man has the confidence of your brother. The smart thing to do is for your brother to never speak to this person again, but that's not going to happen probably, and I'm sorry for that. Protect yourself. Do not loan your brother any money while he is still friends with this con man, because it will just go to some scheme.

I later searched "Jose Uribe error card" on Google and was immediately met with links saying that the card wasn't valuable and you shouldn't buy it for $250,000. This made me more suspicious about the rest of the cards, and I wonder if this could somehow really be an elaborate scam.

Funny isn't it, how the "friend" knew exactly where to go on the internet to establish a high price, and exactly what sites to avoid that would warn people away from this scam.

Again, you might think "why would the con man deliberately choose a card as the lynchpin of the scam where the first four google hits are 'this is a scam'?" That's a feature of the scam, not a bug. It's the same reason why the "Nigerian prince" scams all still say "Nigeria" in them. The con man is looking to quickly discard people who cannot be easily scammed and retain people who do not check for evidence of scams..

See https://www.econinfosec.org/archive/weis2012/papers/Herley_WEIS2012.pdf for an analysis of this phenomenon.

He's only known him for 3 weeks, and he's "super cool"

All con men are "super cool". Charisma is a requirement of the job.

All con men are people you just met. You don't suddenly get conned by your close personal friends you've known for decades.

He just willingly handed over (what he thought was) hundreds of thousands of dollars worth of baseball cards

The con man knows they are valueless.

He's giving my brother a large percentage of the cut, after my brother specifically sells them

"Too good to be true" and "appealing to your greed" are common characteristics of scams.

My brother actually helped him first and he's doing this in return

Nope. The con man is searching for kind newcomers with ready cash to be his marks. The "helping" was a set-up.

There are frequently men whose "car ran out of gas" standing at the freeway entrance near my house, wearing cheap suits and waving briefcases at passing traffic, hoping for a "favour" from some kind, gullible person. There is no car, and it is not out of gas, and the briefcase is empty.

He's a higher ranking guy in his union so my brother says he's more senior and trustworthy

Do a web search on "union mafia connections" and decide whether being high up in a union is positively correlated with absolute trustworthiness. Sure, not all union leaders are mobsters, but that's not the question. The question is "are all union leaders automatically trustworthy?" and the answer is "no".

There doesn't seem like any possibility of the sort of double/take-back exchange that happens in any common scam, it seems like my brother will simply sell the cards, and split the money

Just because you haven't figured out the scam does not mean it is not real! The con man knows the scam and is relying on your brother not figuring it out.

So, regardless of the true value of a Jose Uribe error card, is this an elaborate scam?

Yep.

Or could my brother be on track to make a legitimate quick buck selling these cards on eBay?

Nope.

I’m an avid card collector. The Uribe is a scam, and a well-documented one at that. There is one card in the 1990 Topps set that is “valuable” - the Frank Thomas card that does not feature his name on the front of the card (an actual printing error, as opposed to centering issues). Unless your card is from the 1960’s back (some 1970’s cards qualify, too) or is a modern card that is of a low print run (25 cards or fewer) and is autographed by one of the superstars of the game - like Mike Trout - your cards aren’t worth much.

I've just seen a copy of that rare card on ebay, it's only $5,000 man! We've gotta pick it up, but right now I've got no easy cash, You think you could grab it for me and I'll pay you back in a few weeks? As we're in the union and all you got nothing to worry about, I'll make sure you get paid an' all!.

There's a popular scam where:

1. The scammer gives a target a cashier's check.




2. The target cashes the cashier's check.




3. The target gives the scammer back part/all of the money.




4. The cashier's check is revealed to be a fake, and the target is liable.

The baseball cards sound like cashier's checks to me. This is, they're something that the target may plausibly think is valuable, and perhaps convince others of the same, only to find out that they're forgeries after transferring the value "back" to the scammer.

My guess is that the baseball cards may be fakes. So, it might play out like this:

1. Your brother sells the cards for whatever he can get for them.




2. He gives the potential scammer a 70% cut.




3. The buyer realizes that they're fake and sues your brother.

If the cards are real, then it seems more like an issue of if your brother can find a buyer; and if the sale's legitimate, then I'd guess that the cards are worth whatever the buyer agrees to pay for them. The potential scam would seem to be in the cards potentially being fakes.

Here's eBay's advice:

• 
  "How to Tell the Difference Between an Original Baseball Card and a Reprint" (2016).

And a thread on eBay about an allegedly fake baseball card auction:

• 
  "Fake Baseball card selling on ebay right now for $455.00" (2014).

It's hard to guess what else might be involved in this possible scam. For example, maybe there's another scammer involved who'll act as the buyer, ensuring that the baseball cards actually do sell at a high price, such that the first guy gets a 70% cut of a substantial amount. This buyer can then try to recover funds from your brother after "discovering" that the purchased cards are fake.

This is my take:

Your brother helps someone for one night. They offer him a business proposition. He gives your brother some extremely valuable items, and 33% to sell them. A couple of hundred thousand dollars worth, to be precise. No security, no documentation, no insurance, just the equivalent of "here, take these 200 paintings including a couple of Rembrandts and sell them for me, willya?"

There's a thousand of this card, and this guy happens to have one. And hasn't cashed in, auctioned it or retired. In fact, he offers to pay someone - a stranger, effectively - $80k just to sell it for him.

(Regardless of true worth, this is what he's said he believes it's worth...)

My "uh oh" sense is tingling so much. I don't know what's up, and perhaps nothing, but it tingles. Oh yes it does. Classic stuff, blinding people with greed so they don't consider plausibility.

Risk

Many ideas are suggested in other answers. I'll add this one. I give you a suitcase of $1/4 million cash, or that amount in bearer bonds or land deeds or other bearer valuables. "Can you keep this safe and sell it for me".

Would you feel safe having that in your home, with no docs, no proof, no security? I wouldn't.

All I need do now is arrange a break-in, or perhaps just sit and wait till he comes back to me saying a buyer said they're fakes, or that he wants out. I naturally want my money back. Or I claim these are fakes, your brother must have swapped them. I don't think a discussion is needed if this would work as a scam. The point is, the risk alone of holding what someone claims is that valuable, and tradeable anywhere, is too high for me, without some very careful precautions. It should be too high for him, too, without the same.

One other thing - how did their meeting happen? I'd be mentally reviewing that, to consider if any of it could have been contrived. A conveniently inconvenient car problem, and bing-bang, you have someone you can rope in over the hours they spend chatting. That sort of thing is also common in scams.

Am I being suspicious? Yes - and on those sums and contexts so should he, at least till proven otherwise. Right now it's a million miles from that.

What to do:

Immediate - photo them in high resolution. Get a very safe place for them. Figure who can bear witness what happened or what he said that day when he told friends, and ask them to write brief notes "in case it goes bad" so they can confirm what he said to them and did on the day he got the cards.

Then, if he wants to have a go:

If he's in or near a sizeable town or city, this is what I'd do: Visit a shop, auctioneer, valuer, or similar, and ask them what they think it would fetch.

This does several things. Your brother isn't an expert. These people are. They'll also know how likely/unlikely other options are, or obscure scams, or stolen goods.

If its genuine and valuable, he'll need that kind of advice: where and how to sell it, how to get a certificate of authenticity, how to keep it safe until sold, how to not have it stolen, what he needs from the other guy to confirm provenance (how he got it and how he can prove it is).

If its not genuine, or there is an actual or potential catch, they'll be able to tell him, and advise what to do.

If your brother thinks there's enough of a chance its genuine, enough to be worth working on and not just handing back, he should equally take it seriously enough to seek good advice on 2 of the highest value cards. What he learns on those will be enough info to decide what to do about the rest, and a lot of collectable valuers/auctioneers will do a valuation or a quick comment for free if you walk in the door and ask.

If its dud or a problem he can throw the issue back at the other guy, and not be saddled with it ("sorry, I can't sell it unless I can prove its not stolen, which means I need something that'll prove it from you"). He can explain that the friend may have been duped too, this doesn't mean implying the friend deliberately did it. Just that, without the things the valuer says, he doesn't want to go further with it.

If its good, he will need that stuff anyway, for any possible buyer, as it'll confirm the likely value and make it more sellable.

If your brother doesn't think there's enough of a chance its genuine, then he has no problem. He should walk away now.

It seems to be worth $12. There's nothing special about it. So if all are about $10 then you have $1000 to $2000 dollars there. That is valuable, at least to me.

This is a hype, not a scam. Those cards are probably not worth much. I have known a few people that were willing to offload their collection from the 90s primarily because it’s a hassle and the payout is not expected to be that much.

Your brother might be doing him a favor. He’s not on the line for anything else. Just don’t expect him to make much money.