There is a very disturbing trend in some of these posts. In two or three posts, people seem to suggest that they are owed something for their honesty. I certainly don’t want to belabor the point, but really, all you should reasonably expect for returning someone’s property or money to them is a smile and a thank you. And if you don’t get it, well, that’s a bad on the other guy, not you. To expect a finders fee or reward for returning cash strikes me as equally rude and greedy.
Now to the topic at hand: everyone selling something should understand it’s value, and if he undervalues it, shame on him. You as the buyer should feel no remorse for having taken advantage of him. It’s probably better, though, if you take your major score home and quietly gloat in the car, rather than doing anything in the presence of the seller. Human nature being what it is, no one is going to be happy with being on the losing end of a deal. Granted, no one likes a sore loser, but someone who is barely making ends meet doesn’t see it that way, nor does someone who’s not doing well at the show and is looking at losing money for the day / weekend.
While it is not legal in a retail store to reprice merchandise at the register, if you enter into a discussion of the price of the product PRIOR to purchasing it, then the seller is within his rights to reprice it. Not good business, because it will likely cost you the customer (see the earlier posting where the merchant insisted that the customer take the lower price), but it’s not illegal or unethical, either. However, a train show isn’t a retail establishment, so even that logic may not apply.