This seems like a great idea. Post your books. Get book credits. Request books from other people using those credits and they send them to you. DVDs work the same way. CDs require a $.50 payment. You pay postage when you send, but the other folks pay for postage when you make an acquisition.
Has anybody tried these services?
http://www.swapacd.com/
http://www.swapadvd.com/
http://www.paperbackswap.com/