Half.com is the way to go (I used this site a lot before the ebay merge). I use to make almost $1K every year selling textbooks in college. My roommate and I collected all textbooks from students the bookstore wouldnt buyback at the end of the year, store them during summer. Then place them all on half.com at the beginning of the school year. Your college might move on to new additions, but others are still on the old editions and people will buy them.
There are also sites like bigwords.com that buy textbooks. You get a low price, but its better than nothing if you cant sell it yourself.