I work at my school's bookstore, so I know all about the buybacks. A lot of times you get nothing for the book is because there is a new edition. Other times, the professors haven't requested their books for next semester, so that lowers their value. And the last reason you get nothing back is that some books just aren't being used anymore.
It truly is a pain in the butt. But please don't take your frustrations out on the clerk! We have nothing to do with the prices.
And I know how you feel because I'm a student too.