A few years back I made some quick cash selling books through Amazon.com - if you have any books in great condition that you can sell, it's really, really easy, and - this is absolutely true - I sold my first book within an hour of posting it! I told a friend, and she had the same experience.
Just find a book you have, look it up on amazon.com and click on the button that says "Sell yours here"> you may have to create an account or something, but then it's just a few clicks away to listing books. If you happen to have any popular hardcover books in good condition, you will make the most money that way. Ship media mail. It might even be worth going to a garage/yard sale or 2 and picking up books cheap in good condition.
Hope that helps you out - it's really even easier than ebay. ebay is another way to get some quick cash if you have something someone else wants. I find it easiest to do Buy It Now, personally - then whoever wants it just gets it right then.