ISBN is like the fingerprint of the book - no two different books can have the same one. If I recall correctly it is now a law that the ISBN must be provided. There was a big stink when I was doing my undergrad because my school like many others wouldn't provide them so you didn't know for certain if you were getting the same book if you bought one elsewhere. Plenty of students were signing petitions to lawmakers to get that changed and it happened my junior year.
You're best off buying directly from another student as people have said, more money back for them, less than the bookstore price for you.
Some Universities are serviced by
http://www.bookholders.com/ University of Maryland where I went was the flagship school for that. It's basically a consignment shop, students drop off their books and set their prices online and other students buy them while Bookholders takes a fee. They do ship anywhere so you can buy even if you aren't at a school, but the selection tends to be limited to the required texts for those schools' classes so if your school is using a different one you are out of luck.
After I moved away from there I do a mix of
Amazon/Amazon Marketplace/Used from the school bookstore, whichever is cheaper. Lots of stuff like lab texts are published by the school and have to be bought there, and if things like software keys are required you need to buy used.
I found it just easier like some PP's to expect 100-150 per class and if I got lower that was a bonus. It's impossible and far too depressing to expect I'll get a good deal on any given year.
Also if you are in a STEM field, KEEP YOUR BOOKS. The bookstore won't give you pocket lint for them, and you will most likely keep referencing them. I kept everything and throughout my undergraduate degree I was often referring to my old calc and differential equations texts for advanced courses. Now that I'm in grad school I still refer back to a healthy number of my books. I'd rather keep the $100 book on the chance it's useful then get $10 for it now, and a year or two down the line be out of luck when I wish I could look something up.