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