How do you buy stuff face-to-face with (in my opinion) large sums of money?

I'm buying a used car private party. Its gonna be like $9000. I've only bought stuff cash when I'm dealing with the person face to face.

I've only bought stuff cash of up to about $3000 before. And it was a hassle getting the bank to give it to me. I felt super uncomfortable riding around all day with that much cash in my car. Then the guy I was giving it to was questionable about if all the hundreds were real. And finally, I had no protection after the fact in case the purchase went bad.

So how does everyone else do it? I thought about a money order or a check, but I doubt the seller would accept it because it could be fake, right?