There are 3 things you can do with college textbooks after the semester is over.
1) keep them
2) sell them back to the college bookstore
3) put them on half.com and try to sell them yourself
I recommend doing #3 if you have no intention of keeping a certain textbook. Now, this may not be beneficial if you have a lot of science textbooks because let's face it, a new edition comes out like every 2 months...However, books for your "fluff" classes like that copy of Shakespeare's Hamlet or that random anthropology book that was written in the '30's could be useful to someone else. I found that the college bookstore was only willing to give you pittance on a book you sold back (like a few dollars or so.) Creating an account on Half.com allows you to set the price of your book competitively to match those that are selling on the site. You could actually end up getting almost what you paid for the book if demand is high enough. I've sold a number of books on Half.com and even though you know you paid for those books at some point, it's nice to see the half.com transaction deposit after the selling period ends. Sometimes I forget I sold things so when I go to budget and make sure there is enough money to pay bills, there's the added bonus of money I forgot existed. :)
When you need to buy textbooks, I always checked Half.com first to see if the price was better than my college bookstore. Most of the time it was (despite the fact that I also had to pay shipping). Buying used books also saves you money. If you don't care whether or not someone has highlighted in the book then why not save some dough?