I've made over $600 so far selling off books (and a few DVDs/CD sets) on the Amazon used market. Way more than paid off Delicious Library and the barcode scanner. Woo-hoo!
One odd, welcome side-effect: I've put some books out there that I never finished reading. When they sold, I finished them (and was glad I did.) If I hadn't sold them, they would have sat around forever, with me thinking I'd get around to them “some day.”