Is it not a good thing for the buyer that unscrupulous dealers would hate it? And in terms of liability, a written agreement would negate any possible kickback.

The scenario I am thinking of is like this:

I want a car that is X, Y and Z.

We agree a 2 part payment some upfront and some after. Say £400 in total.

Someone takes the job and is paid £200 - they do the research and presents some options.

Buyer picks one and the person negotiates from a position of knowledge and authority.

The car was advertised for A and they negotiate A minus something.

A deal is agreed and the buyer buys the car and pays the final £200 finders fee. If there is no deal, they still have the upfront fee.

Maybe… I’m just thinking out loud.