Last week I signed up for Paperbackbookswap.com where you can list books you have and then swap with other members. For listing 10 books I received 2 free book credits. I ordered two books from other members which arrived via USPS media mail a few days later. The way the program works is that you get your books for free, and when you send a book you pay the postage.
Its very easy to ship the books - I just sent my first two books and I didn't leave the house. I bought the postage through PBS with paypal, printed labels on my printer, and put the packages in my mailbox. Easy peasy. And now I have 2 more credits to get free books on my wishlist.
While this program is not totally free, its pretty darn close. And unlike scouting around Goodwill, I am able to get exactly the books I want. When you put books on your wishlist you will be notified when someone lists them and get a chance to get the book. If there are a lot of peo0ple who want the same one you get in line for it. :-)
You can also look and see what books are on people's wishlists, so that you can list a book and know it will get picked up right away. (Then you get more credits to get what you want!)
Trade paperbacks and hardcovers I like selling on half.com. But for books that are not worth much on half.com, like mass market paperbacks, PaperBackBookSwap is a great idea. I love how they make it so easy to print your postage. For mass market paperbacks you even get to wrap the book right in the paper off of your printer so there is no bubble mailer to buy. How easy and cheap is that?