It's not even remotely greedy, it's their business. They do the research and know how much something is worth, among other things they also have the exposure to present the item(s) to more buyers in order to get top dollar. The owner should do research and know how much their item is worth before selling it, if they don't do that then I can't really see how it's reasonable to blame the pickers for buying things on the cheap.
It's the same reason why I don't care if I buy a $5000 game from someone and find out they only paid $1500 through some crazy circumstance, I paid the market price, why should it matter what the seller got it for?