I've been using the online bartering service, BarterBee, a while now and I thought I'd share my experiences. (In the interest of full disclosure, that link tells them that I sent you and I should get 5 points for each person that joins and requests an item. You get 5 points just for signing up.)
The way it works is that you sign up and list what Games, CDs and/or DVDs you have to trade. You assign a point value to each item and people use their points to buy them from you. When someone requests the item, you drop it into a Barterbee envelope, sans case, and send it off. When they get your item, you get your points.
Points, the currency of Barterbee, are worth about a dollar each. (In fact, you can actually buy points from Barterbee at that price-point. So I guess it's exactly a dollar each.)
Thus far, my experience has been nothing but great. I've sold/traded off about 18 games and movies and have 4 more in transit. In every transaction I have received more value (i.e points/dollars) than I ever would have if I had I tried to sell the items to a used shop or via eBay. Sometimes as good as a 5-to-1 ratio. Often about 3.5 to 4 on PS2 games.
I've only requested DVDs so far. There's a good selection of XBox games, but hardly any Xbox 360 games. No surprise there, though, the system is too new. There are also tons of games for other platforms. Even Atari 2600 games.
I haven't used the service for Music CDs or PC/Mac games yet. I think LaLa.com, which Annette uses, may have cornered the market of the former. I plan to make use of Barterbee for the latter, whenever I can work up the energy to dig the games out of boxes.
Speaking of LaLa.com, Barterbee could learn a couple things from LaLa. I've watched Annette do the LaLa transactions a few times and it slighlty less cumbersome than BarterBee.