According to MS, there will be some way to buy and sell used games, but it will be done online and they're just not announcing it yet. It sounds to me like they're trying to cut out Gamestop and other resellers by handling the reselling of the games themselves, maybe buying publishers' support by offering them a cut.
Then there's this
But what if you want to bring a game disc to a friend's house and play there? You'll have to pay a fee—and not just some sort of activation fee, but the actual price of that game—in order to use a game's code on a friend's account. Think of it like a new game, Harrison says.