I wonder how this is going to play. I imagine they would like to sell the used copies of in demand games, those games they sell used for 50 bucks, but those would also be the first games to fly off the shelf through this rental service. Also since you can keep the list game you rent before your subscription expires why wouldn't you look for a newish game to keep or try to immediately resell back to gamestop?
Eventually I see them not allowing all used games to be a part of this service.