I think it's the seller's decision to sell something at a certain price. If he's happy with the price tag he decided upon, then I have no problem with getting it cheap.
If he sells something, not knowing a thing about the item, and ask "what will you give for it?", I'll not offer big $$$. It's not my responsibility to point out he has an expensive item. He needs to make that decision himself. He could have looked it up, like I do.
My only exception is kids. When I buy something at a flea market from a kid selling toys, and his parents are not around, I make sure he gets a fair price. If he asks $1 for something worth $10, I'll give him like $5.
But if an adult has a Blue Snaggletooth and asks $2, I'll gladly take that deal, just like we do on Ebay.
"Everything has an end.
Except sausages, which have two..."