From filling out 12 nonfiction book ISBNs on this site, cash 4 books
, they only accepted one of the books I entered, for a cash price of $1.51.
It's an easy site to use (just fill in ISBNs), and they tell you right away what they are buying and not buying.
Other book selling options are at used book stores, on craigslist, amazon, ebay, and paperbackbookswap.com (where you don't get money, but you do get credits).
I'm going to try some fiction titles at cash4books next and see what I get.
Do you sell your books? Give them away? Keep them forever?