This is the best use of any kind of book that is no longer in your use. Donate it to a charity that takes books. They can either sell it and invest the money into their charitable works or simply provide these books to those who need them but cannot afford to buy them. Either way, you are not only finding them a new home but also helping a noble cause.
You can also give your old or out of use books to a library. Again you are likely going to have to give them away for free but they will be utilised by many others as they will be available to everyone at the library who is interested in reading them.
If the condition of the books is good, you can sell them at a bookstore. They generally take used books and though they will not be getting you a ton of money, they will still get you something and moreover they will get you the much needed space that was covered by these books.
If these are course books, you can always sell them at a college book store. They will offer you a pretty low price though. You can also advertise these on college announcement boards and sell them to the students directly. In that case, you will actually have a handsome amount.
You can sell the books on online auction sites. This way you can have a global outreach and anyone anywhere in the world can order them. This will give you a better chance of making some money as well as finding a new home to your books.