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.
That's a really interesting idea. Only problem I foresee is that we might have rentals on that car lined up into the future, though we do have duplicates of most of our cars so we could get around that.
Or, even better, you could just upgrade the next client to a higher tier (unless the client, of course, disagrees when communicating this upfront or the higher tier is fully booked).
I guess the cost of the upgrade to the next client is definitely covered by the sale of the car you had already reserved earlier.
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.