The way they are talking around this is so funny:
From major nelson:
Xbox One and Used Games
We know there is some confusion around used games on Xbox One and wanted to provide a bit of clarification on exactly what we’ve confirmed today. While there have been many potential scenarios discussed, today we have only confirmed that we designed Xbox One to enable our customers to trade in and resell games at retail.
Beyond that, we have not confirmed any specific scenarios.
Another piece of clarification around playing games at a friend’s house – should you choose to play your game at your friend’s house, there is no fee to play that game while you are signed in to your profile.
So, let's read between the lines:
"At retail" - sounds like very specific retailers. And not sell on Ebay or etc.
"while you are signed into your profile" - so this basically confirms if you lend the game to your friend and he wants to play it with his profile, he has to pay.