To provide a source of second-hand books. Because there are currently many people want books. But the experience did not matter where you look. Or the option to enter the site for the second book was not enough.
To provide comfort some people may not be easy to find in stores. Or not enough free time to find books that we want. With the many bookstores.
c) To provide an option to compare. The price of second-hand books Is the book that has not been used.