Here's how it works - you clean out your bookshelves and post books you don't want on paperbackswap.com.  When someone selects your book, you mail it to them.  In exchange, you get to pick books that other users have posted.  So the only thing you pay is postage.

 They have a large selection of children's books. The only issue is you don't know what kind of condition the book is in until you get it in the mail.  It's kind of an honor system.  I've only been disappointed once.