Besides the ones mentioned [and good one on craigslist, I never thought to check there]; there's the amazon marketplace. I've used them before (both buying and selling) and it's allright. They do have a shipping credit that's fixed, so you may end up having to pay for some of the shipping out of your pocket (or if it's a light book, get extra money). My books sold very quick on there, within a week for both of them. Amazon does take out a high commission, but I went with it since I didn't know any better and wanted the books to sell quick.
There's also textbookx.com - it's a small operation and have very reasonable prices; but I haven't bought anything from there yet...
regards,
keyshawn