Search for the book you wish to sell. Depending on your chosen site, this might be accomplished through an actual search, or simply by browsing the listings. If the results show multiple editions, make sure you find the correct edition.
List your item and accurately fill in the information such as title, edition, and possibly ISBN.
Thoroughly describe the book's condition. The more recent the edition, the higher the price can reasonably be. Keep in mind, however, that if there are other listings on the marketplace for the same book, you may be competing with lower prices.
Select a forum popular in the town in which you attended college. This is more likely to be where students find books than the town you live, if it is different. Depending on your location, this will be a site such as Craigslist, Gumtree or even Facebook.
Post your item and make sure the title and ISBN are correctly entered.
Follow the instructions for a "for sale" posting. This will include specifying that you are selling a book and, if the selected town is a large enough area, where in the area you are selling the book.
Fill in the title of the book and the asking price. Give a thorough description of the book, including its condition and whether you have marked or folded any of the pages.
Choose the site on which you plan to sell your book. This will depend on personal preference more than anything else. Some sites will pay you a standard rate for a book immediately, while other sites will sell it on a public marketplace.
Compare offers. This only applies if you've chosen an upfront payment site. The prices may not vary by too much, and none of the prices are likely to compare to what you paid for the book if you bought it new.
List the book using the specific site's instructions. Remember that your books are more likely to sell if you give a thorough description of condition and markings.