The way Gamefly does these type of activities is this; When you trade in a game, you receive "Gamefly Dollars". You use these dollars to buy other games, and to amount of dollars you get will depend on the popularity of the game, and how much demand it is in. but if you check out http://www.gamefly.com/TradeIn/Blocked/ (Gamefly "Trade-In" button) everything will be answered there as well.