List your books for sale on campus for the quickest results. It's a little work making tear sheets, but it will garner you the most immediate sale without the hassle of shipping. It's best to place the tear sheets in the building in which you attended the class that you used the book in. Chances are, there's a student passing the sign that needs your particular book and will be more than willing to pay cash.
Resell your book online at the end of the semester through Amazon, Craigslist or eBay's Half.com. Both Amazon and Half.com are super easy to list on and have an easy step-by-step process for listing books. They will already have pictures available according to the book's ISBN (International Standard Book Number), which is located on the back cover below the barcode. To list, all you need is a bank account for Amazon and a PayPal account for eBay that the buyer deposits their money into. Amazon payments take longer to process, and many times you will not get paid until the recipient has acknowledged that they received the book.
Advertise that you have books for sale through social networking. This is a great option for selling freshman books since you will more than likely have slightly younger friends who are entering as freshmen just as you are moving on. Be upfront with the price and if you like, make a comparison for how much it's going for online so your friends will have a reference.
Trade books with a friend who might have courses you need to take and might need the books you are wanting to sell. You might try writing an ad for your dorm that lists the courses and books you've had and the courses you are taking. That way you don't have to deal with a monetary transaction, and you still save on the cost of your books.