Only buy what the professor considers "mandatory".
Then:
(1) find pdf versions online
(2) buy new off Amazon (take adv of the Amazon Prime student trial)
(3) buy new international editions off Ebay (My Auditing textbook this semester - Amazon $255 (
http://goo.gl/9oKUR7), Int'l $56 (
http://goo.gl/5m72T6))
(4) get no textbook at all
Keeping in mind that this excludes sharing as that is a pain and used textbooks because those are gross and in my experience not much cheaper than buying online. Plus you may be able to resell for higher than a used one.
It's harder to gauge the difficulty of a class at first (check professor rating sites), so go to it then decide if the book is even necessary. Once you hone this skill, you can do the online shopping before classes start. If Amazon prices are similarly outrageous to the campus bookstore (RIP Titles), check for international editions on Ebay. Don't forget to read item and seller reviews on all sites. As for finding it online, take what you can get, then decide how much you want your eyes to suffer. Plus, if your credit card offers cash back or some kind of reward for qualifying purchases, that's more savings. The campus bookstore should be your last resort for buying and selling. Use the FB group for selling.
Source: 4th year Accounting major. Opinions slightly more U.S. oriented.