Every college student knows the pains of buying up textbooks and not being able to sell them back, or at the very least enough money to be worth it. Kindle and other e-book owners are also ditching their novels for the cost-effective space savers. Face it, books are space eating, mold growing anachronisms of our past. Books are great, they're one of the most culturally valuable parts of our society, but paper has to go.
The site Cash4Books offers straight up cash for your (relatively) old books, allowing you to look them up by ISBN to check their value (or lack thereof). They even pay for FexEx shipping (not the box). It's not a bad deal when the books will just get older, more damaged and less valuable anyway. They pay via PayPal only, which has recently increased its fee rates, so some of the payment will be lost.
C4B aims at trading in books printed in 2006 or later, with possible exceptions to rare older books. It's fair enough, no one is expected to take any old book blindly, but give them credit for pre-confirming the books' value. With a large collection of mass market paperbacks, very few of mine were actually accepted, but it still saved me from storing them until I have a garage sale and sell them for a dollar. In some cases, some hard cover books can pay for their digital replacement.
A word of warning for anyone excited about trading in old books: start a pile of books that they will accept, but don't generate the order for a day. You'll find more books at some point and wish you added them in.