I sell a lot of books on half.com, which is part of eBay. I have had good luck. Buyer pays pre-set media shipping costs. When you list the books, the program gives you a range of prices for that book based on the condition of the book and tells you what it is currently selling for and what the last copy of that book sold for and how many are currently listed. You can also set your own price and you don't have to take pictures. You can use the "copyrighted" dust jacket photo of the book as your listing picture. It's really pretty easy.

If you have rare books, or first editions, I would try Craigslist, or an auction on Ebay. You'd probably make more money.