Always do the transaction sitting at the BMV (or DMV, or whatever your state calls their car titling and licensing authority) sitting in front of a clerk. That way there is no problem with the title transfer, left over taxes, notary stamps, etc., etc. No surprises.
Always insist on cash. Period. And get the cash, too, while you're sitting in front of the BMV clerk.
Take someone with you. You need a ride home anyway because you've bought the car with you so the buyer can drive away with it.
If you have a buyer that won't agree to any of these things, tell them (politely) to fuck off. Another buyer will be along soon. Also, buyer from another state? Fuck off, that involves another state government and you can't sit in front of two BMV clerks at the same time. You'll find another, in state, buyer shortly.
This advice formulated over about dozen private car sales transactions, being on both sides, and experiencing a number of mishaps.