If you are a college student, or you know someone who is, they need to know about Amazon.com's Marketplace. Selling used books is easy: you search by UPC of ISBN for the book you already have and list it as used from there. Buying used books is easier because they're listed right along side of the new one:
I listed all of my text books for this past semester on the Marketplace; typically I sell them within a week and I get a few bucks more than if I sold them back to the bookstore. On the flip side, some student gets the book for a lot cheaper than used (especially since I always list mine at the lowest price).
Two of the three books I need for Summer term were available on Amazon Marketplace. As you can see here, our bookstore sells the first two books I need for $133.00 new / $99.75 used and $160.35 new / $120.25 used. Since Summer term typically uses the previous years editions, I was able to get both for $26.00 shipped.
As for the third book, I'll be wandering over to the bookstore to use my employee discount along with the $50 gift card I won for filling out a student health survey.
Greta and I are probably the only people who remember when Varsitybooks.com sold college text books, and I've screwed around with most of the college book websites, but by far Amazon's consistently has the best prices, is the most reliable, and I actually get the book I wanted -- not the prior edition, not an international edition, and not an advance copy for professors to consider.