Check with an employee at your school bookstore for the dates when the store buys back books. For most schools, the last two weeks of the semester are designated as buy-back periods. Most bookstores will only re-purchase a textbook during that time.
Make sure your books are in the best condition possible. Remove any loose papers and erase any notes that are jotted down in the book, if you can. Some bookstores can be picky about the condition of books they will accept for buy-back. They will however, accept books that are still able to be read easily, despite some writing or highlighting.
Go to the bookstore during buy-back time and bring your college ID. Almost every school requires that the seller presents a valid school ID if they are to sell back books. The bookstore employee will ring up all the books and tell you how much each is worth, if anything. Some books may not be eligible for buy-back, if there is a newer edition available or if the book is in poor condition.