Here's what it says on their website:
Joining Swap Tree is free, and here is how it works:
List. Choose. Swap.
Simply list the books, CDs, DVDs, and video games you have.
Choose the items you want to receive.
Then just swap your items through the mail.
Choose From Thousands of Items.
Swaptree's trading engine instantly shows you the many thousands of items you can receive in return for the items you are trading.
No Post Office
Swaptree makes mailing your item easy by allowing you to print a perfect postage label right from your computer. There's no worrying about stamps, weights, or visits to the post office.
Free. Fun. Green. Safe.
On Swaptree you can get the media you want for free, learn about new artists and authors, share your media collection with friends and meet people with similar tastes, all in a safe and green community. Free, fun, safe and green, what's not to love?