A used textbook is one that another student has previously owned, and chose to sell following the completion of the course. You are typically likely to find the best used textbooks for books that have not released a new edition for a year or two. When a publisher releases a new edition of a book the older editions become largely worthless (to the bookstore) seeing as how next semester the professor using this textbook for class will want the students to have the latest edition. It is important to note that often the difference between the two editions of a textbook are minuscule, and many professors are fine with students using an older edition in order to save money (although I always recommend checking with your professor on this before making a decision).
The standard textbook rule that I tend to employ is that used always beats new, except when the used book is in “bad” condition, or has water damage. This is because used textbooks are always significantly cheaper and often come with a few extras (such as a previous owner who may have underlines or highlighted all of the important paragraphs, and may have written down answers to problems in the textbooks margin). Trust me, I know that this may not sound entirely fair, but when you are taking a full course load and have a part time job (to pay for your textbooks) it is good to have any help you can get.
