Let me know if the User Registration Add On would allow me to do this.
Scenario:
-User comes to the site
-fills out information about themselves (address, phone, email)
-this info gets stored in a database (or likely as a wordpress user)
-next, in a new form they add information about a book they want to sell (as a post)
-this gets shown as a post to other users
-the user could then add a second or third book without having to type their address again
-finally, through the back end, I could print a list of all of the books a specific user has for sale and total the cost