Obtain the required reading lists for your courses as early as possible. The more time you allow yourself to find used books and provide time for shipping and handling, the better your odds of finding a good price or getting a copy before all the used ones get snatched up.
Take the required reading list to the campus bookstore to find out the ISBN numbers for each book if they aren't already on the lists. An ISBN number is a universal code specific to each publication and edition, and is located on the bar code.
Look for used versions of your required texts while you are at the campus bookstore. Make note of the prices for comparison shopping later.
Look at websites that specialize in used textbooks. Major retailers like Amazon and Barnes & Noble offer used textbook sales, as do sites specific to textbooks like Textbooks.com and Usedtextbooks.net. Search these sites, using the ISBNs from books. If a book is brand-new, used editions may not yet be available.
Add shipping costs to online prices before you decide on a purchase. At first glance, a used text found online might seem like a great deal, but there are often hidden shipping and handling fees. Add those in and then determine if the online used books are less costly than those in your campus store.
Ask your instructors if only the newest edition of the book is acceptable. Many new editions of textbooks update only a few small pieces of information. If the book hasn't been completely redone, your professors might permit you to find an older edition for less money. If the older edition is acceptable, adjust your search by using the second-most recently published edition as your search item instead of the ISBN from the current edition.
Ask your friends and colleagues if you can buy their old books. This is a particularly useful technique for required general education courses or with other students within your major. If they didn't get around to selling back their books, your peers might be eager to sell them to you at a cut rate.