One step is to head back to the campus bookstore and sell the books back. This is the most convenient approach, but it very likely will be the least economically feasible approach. In most cases, you will get less than 25 percent of the price you paid for the book. In some cases, you might not even sell it back for 10 percent of the original price.
Another option is to try selling the book on an online auction site. This is likely to bring you more money as long as you are offering the book at a premium discount than what you bought the book for. The downside is you never know what kind of offer you are going to get. There is also the extra effort to ship the book when someone buys it.
Instead of going through the whole auction/bid process, you could choose to sell directly to a website that specializes in buying college books. There are many of these smaller sites. Also, larger corporate sites such as Barnes & Noble and Books a Million have programs to buy used textbooks online.
There will always be a steady stream of students taking the same class you took who will need a book. If you are willing to sell the book for less than they can get it at the campus bookstore, then you can make more money off it than you would by selling it back. Put up a few announcements on the bulletin boards in the buildings where the classes will be held and you will undoubtedly get more than a few responses.