What is the easiest way to sell used textbooks? Craigslist like sites are good. If you don’t find anyone there, you can try the websites created just for this purpose. I tried the websites listed below.
Cash4Books: Interface looks good. But, the site pays peanuts for the books. They generously agreed to pay $2 for the book I paid $90! No, thank you.
Bookjingle: Same as above.
BookScouter: This is better than two sites listed above. BookScouter is more like comparison shopping site for used books. When I was browsing bookscouter, I came to know about Amazon’s trade-in program.

Bookscouter List
Amazon’s trade-in program is simple. Amazon agreed to pay $21 for my textbook whereas every other cheap site was adamant about $2-$5 price range.The catch with Amazon is that they give you gift card instead of cash. If you use Amazon regularly, gift card is as good as cash.

All the above sites, including Amazon, have lot of restrictions when buying used books from you. If your book is published before 2008, you are out of luck. Used textbooks sites are looking for fairly newer books so that they can turn it around quickly. So, don’t wait around to sell your used textbooks. Sell them as soon as you get your grades.