You do realize that Gamestop has to do trade-ins the way they do it, or else they'd go completely out of business. They're not greedy being greedy with it at all. All of their revenue comes from used games. If they gave you as much money for trading in the game as they sell them, they would make hardly any actual money. Plus, the trade in value is based largely on the popularity of the game itself. I don't think a game that's only mildly popular should get the same amount of money as a newer, more popular game that more people buy.
It's not about greed, it's just business. Gamestop is already going to be hurting due to their increase in trad-ins, thanks to walmarts new trade in system.