At game stop, anytime you buy USED games you have up to 7 days to return them for full credit, it has been that way for a while, and they also can start to refuse you this deal if it turns into a weekly thing and you are saying that you just don't like any game and it turns into a free rental kind of thing.
And of course they are going to rip us off when we trade, thats their business, it is how you buy games that are normally $50-60 for $35-40, I hate it too but you can't blame 'em