For most people, college is a time where even a couple of dollars can have a dramatic impact on your week, so one of the most valuable skills you can learn early on is figuring out which of the many "required" books are actually necessary for making it through the semester.
Your professors might tell you that you need to buy the most recent versions of the textbooks they conveniently receive royalties from the sale of, but there's no reason to roll into your school's bookstore the first week of school and buy a brand new copy of everything listed on the syllabus.
Based on the second half of my senior year, it is possible to make it through an entire semester without purchasing a single textbook, but if that isn't an option, your best bet is normally buying the cheapest used copy you can get your hands on.
The reduced price tag is definitely one of the major perks that comes along with buying a used copy of your books, but it's far from the only reason.
I know no one wants to buy a book that's filled with notes and excessive highlighting, but every once in a while there will be a note or a doodle from a previous owner hidden inside the pages that can make the entire purchase worth it.