If you're a student, you both need to buy textbooks and figure out what to do with your old ones. In most cases, you probably won't look at the textbooks very much in the future, so you should sell them quickly before they depreciate in value. The textbook publishers try to come out with new ones every few years so as to keep sales high. Once the new edition comes out, the prices of the old ones drop dramatically, so don't delay.
Where to sell Textbooks
Another Student - Again, this is the simplest option if you can do it. You avoid paying fees to any site, and you don't need to bother shipping the book anywhere. You can post an ad on college classifieds site, or put up a sign in the classroom that will have the same course that you took. Then you meet the buyer, exchange the book for cash, and you're done, without any fees involved.
Amazon or Half.com - If you can't find another student, this is second-most profitable option. You enter the ISBN and condition of your book and it gets listed on the site. Amazon is the most popular site for buying books, so its probably your best bet. However, Half.com has slightly smaller fees, so it might be worthwhile to sell popular books on it. These are the exact fees:
Both sites charge 15% of the purchase, but Amazon also charges an additional flat-rate of $2.30 for every book sold through their site. Meanwhile Half secretly takes part of the shipping commission, so you only get $2.64 or $3.07 as a shipping credit, depending on whether its soft or hard-cover. However, Half does lower the fees for books over $50.
After the book sells, you need to ship it to the buyer within a couple days. Normally, the cheapest way to to this is with media mail. Both Half and Amazon let you buy shipping labels through their site, which is the most convenient option. A single book normally costs $2.41 - 3.64 to ship, depending on the weight. You then print out a shipping label, stick it on a padded envelope, and mail the book away.
This table summarizes the costs of each site:
Price * 85% – 2.34 fee + 3.99 shipping credit
Price * 85% + 3.64 (hardcover)
Price * 85% + 2.07 (softcover)
Money received on a 2-pound hardcover $30 Textbook after shipping costs.
Buyback Sites - Another option is to sell your book to a buyback site. This normally isn't worth it though, since they pay much less than if you just sell it yourself, and you still need to ship it somewhere. The exception is in the rare case where you get a decent amount through these sites. The book must be in very good condition though. As with buying books, you can compare buyback prices with a price comparison search, such as on DirectTextbooks, or NYtextbooks. Amazon Buyback occasionally offers a decent amount of Amazon credit for a book, so that can be a good option if you'll anyways be buying from Amazon in the future.
Campus Buyback - This is the most convenient option, if its offered on your campus. Be careful not to get ripped off though. When I ran a buyback, I payed 60-70% of the book's used selling price on Amazon. Amazon than took 15% and other fees. Then there were other costs, such as for envelopes and barcode scanners, and some books didn't sell, so I had a small but reasonable profit in the end. Just check the price on Amazon before selling your book and make sure you're getting over half of it. Otherwise, its worth it to just sell the book yourself online.