I recently came on an interesting site for swapping books, CDs and DVDs.
Swaptree was founded in 2004. You join, enter UPC codes of the articles you want to exchange, and their site attempts to find a comparable swap for you. You can print Media Mail postage directly from their website to attach and mail the items from a post box/drop (or your home outgoing mail if you have it) rather than going to the post office.
This seemed like a good site to share on Earth Day.