I have heard many people complain that they cannot sell books. Many of them throw them away. I sell books rather well on eBay. I will tell you why.
With the exception of vintage books, the market is typically saturated with almost any book I get my hands on. It is difficult to compete in an over-saturated market. So, in order to combat that, I sell my books as lots. If I have several books by the same author, I will sell them together. If I have several cook books, I sell those as a lot. When I do this, I make sure to include the word "Lot" in my listing's title. Be creative in compiling lots of books. Keep them somewhat categorized.
Also, use Media Mail to ship books. Books can be very heavy and Media Mail was designed to combat that. It is considerably cheaper than any other shipping option. And, if your customers are paying less for shipping, they will be more likely to pay more for your books.
This isn't full proof but it can help. And if you would like a great alternative to this approach, please vote on this guide, then visit my guide about selling your books on half.com.