I always buy a used book from Amazon. Anyone paying full sticker at the campus book store is getting robbed. Last years edition is almost always fine (unless the instructor is using the accompanying courseware - but generally my school has stayed away from that). The ebooks especially are a bad deal since you just rent them and can't re-sell them.
For my statistics class this fall the text is: Statistical Techniques in Business and Economics 16th Edition, ISBN 0078020522. $292 at the campus bookstore, $248 new at Amazon, $227 used at Amazon. Previous edition (15e - approved by Instructor) used at Amazon? $44 bucks.
I've done this through all 4 years of school and am about to graduate. Only twice did I have instructors require the on-line courseware. At $100 bucks a pop, it was still cheaper just to buy the code by itself and go with the older edition book.