You can sell them at
www.half.com or abebooks.com. My daughter used half.com for her college textbooks. Half is a part of ebay, but you can buy and sell lots of books. Keep in mind, as you are pricing your books, that your charges for shipping may not match the actual shipping charges, so you might need to think of that for the actual price. This is a great site, and you will need to see what the other books of similar type and condition are going for - you probably won't make a killing, but you can get rid of some of them.