Customise script

Hello all I already have a site running but want the script changed its a joomla site running as-manager script, the site is a car swapping site and want the script so user can make offers to each add. i.e User A has a car add posted and user B makes a offer from User A ad. Now this offer can be a straight swap for a car or part swap i.e car plus cash. I also have other things i want added but want this done first. My site is (Posting of contact information on projects is prohibited by Freelancer.com Administrators. This can cause account suspension or termination. Removed by the admin.) Thanks