The problem is there's no way to prove that you're doing that and not a scammer; you're almost better off just asking someone who bought one recently what they did.
(What you're suggesting is basically Nothercastle's Used Cars)
Except it’s more like a buyers agent or a doula. Someone that’s navigated the process many times to assist on behalf of a buyer. Upfront payment no commission or kickback could even put that in the contract.
I do think these people exist I’ve seen them advertise services in specialty car forums but it’s very niche.
You see it more frequently in certain luxury vehicles where the price is part of the prestige, usually to where the equation is a bit more managed, e.x. they can set base price higher and have fewer random 'tire kickers' to deal with.
(What you're suggesting is basically Nothercastle's Used Cars)