They need to have a carlink type website and you will then be able to allow each other access to your respective sites as well as you would have access to a common area where all dealer to dealer deals will be placed by all participating dealers. " This area is closed to the public" and you are not forced to use this facility.

" The choice always remains yours"

You are in total control as to whom you give access to your site.

Hint - Broaden your sales force by allowing  others (freelance salesman)  to view and sell your vehicles with no basic salary etc overhead.