Half.com

Full review
I am both a seller and a buyer at half.com and I am VERY pleased with my experience. I heard about half.com while I was at college and started using it to purchase books for my classes. You can save A LOT of money this way! Don't ever buy books at a college bookstore! I've found books on half.com for more than 50% off what you would pay at school. I've also purchased movies on half.com and have been very pleased with this. It's by far the cheapest place to get used movies/music/books. There are seller ratings so you can see if the seller is reliable. Shipping is cheap with media mail and is pre-set by half.com so you don't get ripped off like you do sometimes on ebay.
Selling on half.com is very easy. You don't even have to take a picture of your book/music/movie. For books, you can even search by ISBN number. There are a few different condition ratings to choose from that are very detailed and easy to decide on. half.com helps you choose a price and gives you a suggestion, but you set the price yourself. You are also reimbursed for shipping. There is a small commission, but no listing fee like their partner site, ebay.