Its called SwapTree.com
It's still in Beta and only very recently opened to the public and its still working out some quirks, but check this out...
You get on there, and you list all the books, the DVDs, music Cd's and video games that you have that you want to trade. Then you make a list of all the things you would like to OWN. Then you sit back and wait, and when someone who has one of the things you want, wants one of the things you have, the site contacts you with the possible trade! You accept (or reject) it, and you pay to ship the one you own, and then you get the one you want in the mail!
Now what else is cool is this: Say you have a book that Joe Schmoe wants, and he has a game that Jane Doe wants, and Jane has a DVD that YOU want! Well! The system can work three way trades out and will contact all three of you. If you all agree, you pass to the left and take from the right, and all is well with the world!
Just like eBay, you leave feedback on how the trade went, and people use that to decide how much they trust you to do a trade.
Everything is one for one, so you don't put anything up for trade that you wouldn't be willing to pay for shipping on, or trade for ANYthing on your want list (it sounds like they frown on trade rejections for no really GOOD reason). If you have the whole Season One of Lost and you have a desire for a slim poetry book on your list, then you should be willing to trade the 4 disc set for the one little book.
They don't accept videos for trade, but want to add audio books in the future. You can swap discs without the original box, if you describe its condition that way, but no bootlegged anything.
Oh, and did I mention its FREE!!??
At this point I am really disappointed that I ditched all my spare books before moving across country, but you can bet I am going to be scouring the garage sale bins for anything popular from here out!!!