Have you ever used an agent to shop for a car?

I’m in the market for a new car (or a not-very-old used one). I know I’d be hopeless at negotiating and bargaining. I have no knack either for bluffing or for cutting through someone else’s bluffing. I tend to talk straight with people and expect them to do the same with me. I’d be no match for a car salesman.

I’ve heard of people who’ve hired someone to shop for them and been very happy with the result.

How does that work? What kind of terms are usual? How do you find someone to do that, and how do you know whether the person is reliable?