Yeah, you can do it with Guitar Hero transfer codes pretty easily... or so I heard...
How about this scenario:
Guy posts that he is really in need of money and needs to sell some things. One of the things he lists is say Dragon Age Origins for 360. You offer him $25 - not a rip, but not near market value. Does the seller have any right to be upset? After all what is more important - the seller getting near market value for his game, or the seller getting money, which he said he very badly needs?