Listing at the end of August, the end of December, and the end of March are usually your best times. Sometimes the end of June can also be good for people doing summer school.
If it's a big, expensive science, engineering, or math textbook that you're pricing significantly lower than the bookstore would sell it, chances are it will sell.
When I was in school I could make $50-100 a month doing textbook dealing via Ebay. The biggest tip I can pass on is to keep your books looking as new as possible--then you can sell them for more. Also, if it comes bundled, make sure to hang on to all of the things it was bundled with. That sells for more.
Good luck
