To get the best return from your textbooks, direct sales (e.g. to other students) is best, but also hardest since you need to find customers. Second best is soliciting the assistance of a third party (e.g. Half.com) to find customers. With Half.com you can expect to lose about 15% in commission to Half.com plus a little in shipping (since the fixed amount they "reimburse" for shipping usually doesn't cover the actual cost).
The third option is easiest but will yield the lowest pay per book, but is easiest because you can sell all your books to one company that usually pays for shipping. To help you find the best buyer, some comparison sites exist. Here are a few with their advantage/drawbacks listed:
Be careful who you sell to - BookScouter lists reviews.