FWIW - I did my last purchase through a broker. They took all the details incl paint depth checks on my 4Runner. Called back 2 days later with bids from dealers. I found the truck I wanted and they negotiated $1k off ask price (was already pretty decent price). And we did a 3 way transaction where I sold to broker, broker bought from seller dealer and sold to buying dealer. Broker fee was $500. Not sure if they also took kickback.
But I saved $3k in sales tax. And the broker was fine with private sellers or any other deal I wanted to setup. Was also willing to negotiate on private sales for me. But the one I wanted was at a Honda dealer.
I was skeptical. Actually very skeptical. But it worked great. I'm my case it was mostly the sales tax value that made it well worth it. And it was super easy. A 15 minute car inspection with no commitment up front. Then on the back end 20 minutes of paperwork on the transaction. No sales pitch, no add ons, just a wire transfer, sign the papers, get the keys.
Would use a broker again in the same situation.