With college textbooks costing hundreds of dollars each semester, buying used college textbooks is a popular way to help control the cost.
Finding Used College Textbooks
In The Bookstore

Bookstores distinguish used books from new books with a sticker on the spine, typically yellow. Otherwise, they're placed in the stacks along with the new books. Used college textbooks can be snapped up quickly, so the earlier you shop for books, the better your selection will be. Some stores make more of an effort to stock used books than others, so if your college has multiple bookstores, it can pay to compare the stock.If you're purchasing used textbooks at your college bookstore, the top two things you should consider are binding and markings in the book. First, to check the binding, make sure the book cover is firmly attached. You can usually test this by holding the front and back cover in opposite hands while looking at the gap between the spine and the pages of the book. The less the pages droop, the better the binding. You should also open the book and check for places where the pages are pulling apart or actually falling out. It's not a bargain if the book falls apart two weeks into class.

How much highlighting, underlining and other writing you are willing to accept in a used book is a matter of personal preference. Some find excessive highlighting distracting and want to decide for themselves which passages in a book are most important. Others would rather enjoy the price discount than worry about pre-written notes in the margin.
Online

Any online merchant selling used books, such as Amazon.com and Alibris, should also offer a selection of used college textbooks. In addition, several sites have been created especially for the college textbooks trade. Bookbyte.com, Campus Book Swap and eFollet are just a few of the options. You can ask your professors for a list of the required texts before the semester starts, or you can visit your local bookstore to create a shopping list and then search online.

Of course, you're taking a chance on the condition of any used book you order online, but you can still use the capabilities of each site to your advantage. Read all descriptions fully, and look at seller ratings if available. If sellers have sold books of questionable quality in the past, it should be reflected in their feedback. Always make sure you're ordering the correct edition, and ask questions of the seller if you have any doubts.
Other Sources
Check bulletin boards for flyers from students selling their own textbooks.
Ask around in your dorm to see if anyone has just taken the courses you're going to take next semester.
Find other students in your major and ask if you can swap books for required courses.
If your required reading includes books that are widely available, like literary classics, you can also search used bookstores, yard sales and thrift stores. This method requires a little luck, but if you have to read Hamlet, there are numerous copies out there. Before buying a different printing of a novel, though, make sure your professor hasn't assigned a special edition including critical essays or other resources.
The Cycle Continues

Now those stacks of used books have to come from somewhere, so at the end of each semester, bookstores have a "buyback" period where they will purchase your books from you. Unfortunately, they only pay a fraction of the original price. Still, most stores base their buyback prices on the cover price of a book, so if you originally bought the book used, you'll get a better return on your investment.Buyback prices vary widely, depending mainly on whether the book is being used at your school next semester. If it is, you'll get the maximum amount, usually topping out at around half the cover price. If it's not, you'll be offered a much smaller amount, according to nationwide demand for the book. Some students choose the immediate money, others take a chance on waiting until another semester when the class may be offered again, and still others decide they would rather keep the book as a reference than get five dollars for a fifty-dollar book. Unfortunately, if there's a new edition of a textbook coming out, no store will buy the old edition back from you, and you're stuck with it.