Sweet fucking Christ, college textbooks are motherfucking EXPENSIVE. My psych textbook alone was over 200 fucking clams! Wiped out my bookstore credit in one swipe...oy vey.
When it's time to sell, sell it either on Craigslist, or to other students directly. The school bookstore will give you maybe 10% of what you paid for it.
If you charge $100 or so, the people you're selling to get a major discount on the book, and you still get half your money back (which, again, is WAY more than what the bookstore gives you when you "sell" your books back to them).
If there's a Snoozy's near your school, they tend to give you about twice what the school bookstore does at buy-back (which still isn't much), and to offer used books at a much better price (although new copies tend to cost just as much). I only went to the school bookstore if Snoozy's was out of a book I needed.