OK, I've been gone a while and haven't had time to make through all of the pages on this thread yet (I'm working on it!) so I don't know if these links have been posted yet, but these websites have saved me tons of money.
If you are a big reader (like I am) and don't have the money to spend on new books, this is great. I will buy books also from thrift stores, but this way you have much more luck finding the books you want. I haven't spent more than $2.50 for a book in over a year.
The first is Paperback Swap. (I actually found out about this site from one our AFF members signature!)
http://www.paperbackswap.com/index.php?n=2
The next is SwapTree. You can also trade DVDs, CDs, and video games at this site.
http://www.swaptree.com/WebFrmUserHomePage.aspx
All that you pay at either website is the postage for shipping your item!