[swang]

Whenever something like this comes up (item bought at flea market and the like sells for millions), this question or similar gets asked. But why is that? The purchaser bought the photo at $2, which is what the store priced it at or was willing to part with the item. That's the only obligation the purchaser had. Pay the store money.

[Tenhundfeld]

I think you're right: there's zero legal or moral obligation to share your fortune with them.

But I understand why this idea comes up. Put yourself in the shoes of the "junk shop" (from article) owner or flea market vendor. We're not talking about a high-end antique store. The seller is likely not wealthy. They're going to be devastated that they let this opportunity slip through their fingers.

A tip or kickback of, say, $25,000 would only be 0.5% of your pre-tax gains, but it would likely be a significant windfall for them, let them pay off some debt or take that vacation they've always wanted to. It's a nice gesture and would help take the edge off that anger and regret they'll inevitably feel for the rest of their life.

However, before I did that, I would consult a lawyer. On the one hand, sharing a small piece of the pie with them might give them something to lose and make it less likely they'd bring a frivolous but possibly expensive-for-you lawsuit against you – by an attorney who's working on commission and betting that you'll settle. On the other hand, giving them a cut might open the door for some line of reasoning or set some precedent that they're legally entitled to a portion of the proceeds. I don't know. That's why I'd consult a lawyer. :)

[gknoy]

Easy solution: go back and buy another piece of junk for $25k. ;)

[biot]

What if a blind person were selling a box of old useless foreign currencies for $2 but when you look through the box you discover there's a couple of $1,000,000 US Treasury bonds (valid, cashable) inside? Do you quickly snap it up, thinking it's a fair trade because you're paying the asking price? Or is there a sense of having taken advantage of the blind person as they couldn't see the value of what they were selling?

I'm not suggesting any sort of obligation towards the junk shop; it's an interesting thought exercise of where one draws the line when there's a knowledge gap between buyer and seller, particularly one involving six orders of magnitude difference in value.