Once you have read a book, you may not be interested in keeping it. If you have only read it once or twice it is probably in good condition. Used books in good condition can be sold for some extra cash. Bargain hunters who do not want to pay full price for a book are always on the look out for good quality used books, so there is a market for used books. You just have to know where to sell them.
Collect all of the used books you are interested in selling and make sure they are free of ripped pages, water marks, torn covers, and writing or highlighting within the pages. If any of your books have those defects you will have to resort to selling them at a yard sale.
Contact your local used book store, if you have one, to see if they are currently buying any books. This is the quickest way to sell your used books because you can drive over and have the owner make an offer. Unfortunately, there are not always used book stores in a town.
Go to CKY Books, or Cash4Books website and type in your used book's ISBN number. The website will come back with an offer. Both websites are legitimate and will allow you to print out a prepaid shipping label to ship your books to their warehouse for free. Once they receive the book in good condition they will mail you a check.
List your used books on Amazon if you are not happy with the prices offered to you in step 3. This could take the longest for you to sell your used books, but it allows you the most freedom. You can set your own price, but still get massive exposure as Amazon is such a well known site.
There is always eBay if you are not happy with any of the other methods.
Don't lie about the condition of your used books. Customers who don't get to see the book before they buy it will want their money back if the book isn't as you described it.