Ahh I remember the good old days of price gouging. I once had to buy a basic accounting textbook for $125, new, as they had just "updated" to a newer version. At the end of the semester, the bookstore said they weren't buying it back as the text had been "updated" yet again.
I can see this reasonably occurring with science textbooks, as new discoveries are being made, theories are being updated, etc., and having an up-to-date text is necessary. Basic maths, literature, and the like haven't changed much over the years, so in those cases it's not much other than a money press for the publishers and the bookstores.
YMMV, but I'll point out a theory upon which I lighted when I was an undergrad. At my school, your cost of books was somehow directly proportional to your major, or more precisely, how much money you were expected to make when you graduated. Engineering and business students paid the most (both per book and overall), and people in my field (BA English/Creative Writing) and the like paid the least. During my junior and senior years, I don't think I ever paid more than $200 total or so for books, and most of them I ended up keeping.
As for selling your books back when the term is over, just find out what price the bookstore is selling used copies, and advertise around campus yourself at $10 under that.