Contrary to what I have read here, I think "Pawn Stars"

[pipoman]

63. It's a different kettle of fish..

I have done exactly what you describe many times over the years..a few times I have actually went back to the seller at a later time and handed them some more money..usually because such a move will make the seller happy and guess who the seller called next time they picked something up they thought might interest me. If you open a used bookstore, OTOH, and advertise to buy books. Someone comes in with those same Roswell books. You ask them what they want for them, and they tell you $30, then you ponder incredulously, and bounce back with, "they're really only worth $20 to me (knowing full well they are worth $500) then, you are an expert taking advantage of someone. If a person walks into your bookstore with the same books and asks you what they are worth (you know they are worth $500) and you say they are worth $20, you may have at least opened your self to civil liability and possibly committed a criminal act and are making an obscene profit because of that..