Many textbook vendors also sell used copies. But be sure to search by exact ISBN number to ensure that you are finding the correct edition. Purchasing the wrong edition can be a costly mistake. If you don't know the ISBN number, look it up at the Paper Tree Bookstore website first.
Also, make sure that any required/corresponding CDs, DVDs, etc. are included with your book if you are not ordering from the Paper Tree Bookstore. Sometimes these are missing from used textbooks. Also, know that any keycodes that are required to access online supplemental information may no longer be valid with a used book.
Finally, allow enough shipping time to receive the book before class starts.