Perhaps, if you have not already, put in a "buy this car now" feature into the app for the dealer and the clients. It should spring up after a certain amount of time, and/or at journey end.

That way the dealer can use this as a way to manage clients trying before they buy, and every rental client is a potential buyer.

Obviously allow the dealer to make offers to the client, but non-obviously allow the renter to make offers to the dealer. And obviously take a (small to start) commission.