Where can you trade in old books for money?

Find a used book store. Most will give you cash for books that they feel have value. Don't expect to get a fortune for them! You can get an idea of their value by visiting some online sites, such as Barnes and Noble's Out of Print area. You can see what they are selling for. In general a dealer will give you 25 to 35% of what they can sell a book for. If it is really valuable, you'll do better.