I just joined this "book club" -- you can find a link over in the side bar. You can register for free at the moment. (And if you register via the link here on my blog I earn a credit for referring you -- so I hope you'll at least check it out.)
They'll give you 2 credits for posting your first 10 books. I posted 13 tonight and one is already on hold because someone had Wish Listed it!
Basically, you earn a credit for posting and mailing out books you're ready to trade and you spend a credit for each book you order. The only cost to you is in mailing the book out, going media it's usually under $2.50 which is cheaper than a second-hand book store. Contrary to the name, they do except hard cover books and audio books (tho' the audio books require 2 credits).
I'm really excited about this as I go through books so quickly and perhaps this will encourage me to let books go rather than hoard them -- something I'm sure Mark will be thrilled about!