After moving a few times (slow learner here) I learned to keep only the books that I actually re-read or refer to. When I upped and moved house again a few years ago, I used paperbackswap.com in the year prior to offload *lots* of my fiction and unused cookbooks/nonfiction.
With paperbackswap, you put a wish list up and when you start posting your books, matches against other's wish lists get chosen. You then mail off your book (your cost). Paperbacks and hardbacks are worth 1 credit, audiobooks are worth 2. As your wish list matches come up, they mail the book to you (their cost). Matches are made on a first come first served basis. For me, it was a great way to thin and refine my collection. If you want to see if particular books are even wanted, you can put up a "Reminder List" which doesn't post the book but allows you to see demand. If there's demand, you can then post the books. While this is not quite "selling", you do at least get equivalent credits in return for sending books. By the time I moved, I had lightened my load by over a hundred of books and only gotten under 20 in return (I was *far* down the wish list for some of them). Even now, years later, I still have over a hundred credits left - I treat them as "free" books for me (well, at least prepaid) and still only get the books I'm pretty sure I will re-read.
As for selling, textbooks are just about useless unless it's pretty new or it's rare. Look them up to see if they're wanted, and if not just recycle them. I have a local used book store I could go to but have just about run out of books he'd be interested in. Other options for me include donating to the library (get your tax receipt) or Goodwill. An option I haven't tried but is fairly popular is to sell on Half.com - not sure why as I do eBay, but for me books are just easier to donate or sell in person. I do have some rare chapbooks that will eventually be auctioned off, once I wade through eBaying my Mom's figurines and Christmas stuff.
