PaperbackSwap.com is a great site for swapping books with other people. The intention is to make your books available to people, and you can ask for books from anybody else on the site. This works out pretty well, and the service has grown a lot.
I have noticed that people occasionally post textbooks and other books that have a decent value. My guess is that the members don’t know that their books are worth anything, so they are willing to give them away in exchange for books that sell for a penny.
Using BookScouter, you can enter their PaperbackSwap MemberID and use their API to retrieve a list of their books. It then displays the approximate value of each book and provides links to look up the value of each book on BookScouter.
The page is pretty basic now, but if there seems to be any interest in it, I can spruce it up a bit. Please post comments here or Contact Me if you have any suggestions or comments.