My 4 years in college I was always dumbfounded as to how lazy and sometimes how uninformed students are when it comes to selling their books. I had many friends who kept their books for no particular reason besides laziness and some who sold them back to the school for 20%-40% of original price! Textbooks are expensive, at $75-$150 a pop! I never sold them back to the school and I never kept books for the heck of it unless I knew it was a book I would reference back to time after time.
Selling your textbooks online is the way to go. Half.com, ebay.com, amazon.com and textbooks.com are all sites that you can get up to 90% of what the book was purchased for. Just this past month I sold my last 3 college textbooks, all at 70% or more of what I purchased them for. Somewhere in America, someone needs your book, even if its two editions old! By selling to the school, students are leaving money on the table. Why not maximize? It's very easy to sell on all of those sites mentioned too.
Another way someone can profit off of these students who don't sell online is to buy the books directly from them. Here's how: Do a bit of research on what books are selling like hot cakes online, then, find out what courses at your school use those books. On the last day of class, wait outside these classrooms with lots of CASH in hand and buy them at rock-bottom prices! Sell online for $20,$30, even $50 bucks more PER BOOK! Great way to make some extra cash.