Gamefly will most likely take a big hit soon. More game manufacturers are requiring an activation code for the multiplayer portion of the game. The code can only be used once. If you were to sell that game to someone else, they will need to purchase a new activation code or they will only be able to play the single player campaign.
my understanding is there is no physical copy of the game to sell to someone else, http://www.wired.com/gamelife/2011/08/gamefly-pc-game-rental/