A good textbook customer is someone who is going to need it in the upcoming semester. You can get word out that you are selling your books by posting a notice on the bulletin board in your residence hall and in the building where the classes are held. Another option is to tell your friends who may know people who will be taking the course. If the class may be held in different buildings, make sure to post your ad in several areas. Your school may also offer an online bulletin board, which will get the word out to more people.
Sometimes bookstores will buy back your books. It's good to check out the bookstores as close to the end of the semester as possible, because stores usually have a quota on how many they will purchase. Some smaller schools may only have one bookstore, which limit the opportunities for sales. However, if you have access to a school in a larger area, you may want to talk to several different book stores that sell textbooks in order to get the best deal.
Websites such as amazon.com, ecampus.com and textbooklink.com allow students to sell their used books online to students around the world. Also, there is a category for books on craigslist.com. This opens up more potential buyers because many times the same books are used for similar courses at different schools. Some sites may offer different prices, so it may be good to check around. Also, sites may differ on shipping charges.
You may not have any local buyers if the professors are requiring different books for the course. This happens when a new edition of the same text is offered, or instructors change. This is when online sales may be the way to go.
Also, bookstores and buyers may not want your books if they aren't in good condition. It's always good to take care of your books if you are considering selling them when you no longer need them.