Private sales fall apart on paperwork far more often than on price. The negotiation is usually done by the time anyone discovers the title is in a previous owner's name or the lienholder never filed a release.
The core set is short. Title, in your name, physically in your possession. A bill of sale with both parties, the date, the price, the VIN, and the odometer reading. A lien release if the car was ever financed, even if you paid it off years ago. Current registration. And an odometer disclosure, which is federally required for most vehicles under twenty years old.
The two that most often hold up a deal: the lien release and the title itself. Lienholders are inconsistent about filing releases after a loan closes, and plenty of owners discover a decade later that the lien is still showing. Check this before you list, not after you have a buyer standing in your driveway with a cashier's check.
Then there is the folder. It is not legally required and it is the reason you get your asking price. Service records in date order, the original window sticker if you have it, the owner's manual, spare keys, and any documentation for modifications, including the parts that were removed and kept.
One thing worth doing that almost nobody does: write a one-page summary at the front of the folder. Ownership dates, mileage at purchase and sale, major work with dates, known issues stated honestly. A buyer who reads an honest list of known issues trusts everything else in the folder. A buyer who finds an issue you did not mention stops trusting all of it.
Keep your own records where they can’t go missing.
MotorStable holds the service history, the registration, and the documents, and hands them back as one report when you sell.