bookbyte from my last experience.
I assume you mean USED textbooks. On line is not the best place to sell as you have to handle shipping, packaging, tax, refunds etc... Students want to buy those new books that sell for $100 for $2 or less used. The best way to unload them is on campus at a book sale. Textbooks are very difficult to sell as they have a limited audience. The trick is to sell them cheap.
I don't sell my textbooks, I rent them to students actually. There's a nice site that lets you do this. They're called Big Mama (http://bigmama.ca). They let me send my textbook to any student across the country. They rent it for like 120 or so days and it gets returned to me. Then I just rent it again.
I quite like it TBH. Best method that I have ever had in trying to get rid of them yet keep them.
Heres a method that beats everything
Amazon textbook buyback, they pay for your shipping and you get an amazon giftcard straight to your account
kijiji
First thought was to use Amazon, but they only give cash rewards, when I only want the money
