Valve adds a used game marketplace to the Steam service! Because Steam servers are used to validate games as licensed by particular accounts, Valve has been in an excellent position to offer easy migration of games and game content for some time.
Recently Gamestop, Amazon, and others have provoked the ire of developers such as Mark Rein, David Perry, etc. due to the perceived loss of revenue for developers.
Valve's offering enables developers to assign a resale tax, e.g. of 20% of a used game sale. It also assists with market price point adjustment, since a developer can list games at a full price, and the used market will auction off games at the highest price they can under the new price. As games trickle through the used market, developers continue to receive the highest revenue the market is willing to offer. Nice.