I have sold a number of cars person to person, ebay, craigslist, etc. After the money and the title, in the US, the most important thing to prove that you have sold the car (besides the title) is the bill of sale. You must have a copy signed by you and the buyer, and they must have a copy, also signed by both parties. It only needs a couple of different things on it. Who you are, who the buyer is, the vehicle VIN, the mileage of the vehicle, vehicle description and the date AND time of the transaction. Some states require this to be notarized, some don't. Every state has different rules on if the license plate stays with the car, or the owner. Emissions may be the responsibility of the owner, or the buyer (I know, emissions, haha, but usually includes a VIN check and some other safety checks.) Out of state buyers will need this bill of sale to register the vehicle, or even to get temporary tags.
And keep the bill of sale forever. I have had a buyer that never registered the car. They started getting parking tickets. 3 years later, California sent me a really nasty letter. I was able to prove who I sold the car to and when, just by faxing the bill of sale to them. They then went after the person who purchased from me. He was a pilot, so apparently knowing how to fly a plane does not make you a responsible citizen.
G.