We love digital distribution services like Steam. They provide a great place that keeps track of your games, while you don't have to keep track of physical media or serial keys.
Even though there isn't much of a used games market for PC games, the way digital distribution works today makes reselling something you own impossible. Wedbush Morgan analyst Michael Pachter spoke in an interview with NowGamer about some kind of Steam trade or exchange service: "Steam gives gamers enough other stuff so that they don’t resent the fact they can’t trade in their games. And you know, name all the Steam games that you’ve purchased that you’ve traded back in to somebody else for credit. Steam’s about to let you do that supposedly, you know like trade and exchange, but they’re going to take a fee from it."
Before you celebrate about how you may be able to send your digital games back into the Steam stream for someone else to enjoy, Valve's Doug Lombardi killed the notion with the following statement to Blue's News: "Untrue. We've never met with Mr. Pachter."