Half.com is the best choice, except you probably won't sell them until next fall semester comes (slight chance if someone needs them for a summer class). It doesn't cost anything to have them up there, so you might as well list them now.
You should never buy books from the bookstore if you can help it. Ask your professors before the semester starts and get the books ordered before the rush on Half.com. You'll get better prices and faster service.
I sold most of my books back on Half.com. Some of my elective course books, I bought there and resold for $5-10 less than what I paid. Pretty good for basicly renting the book one semester.