You’ll need the number plate of the car you want to sell, as well as details of its history including previous owners, mileage and the condition of the car. You’ll also need your personal details to hand so that your chosen buyer can contact you.
At the point of sale, you will need your V5C registration document, you won’t be able to sell your car without this. If you don’t have one you can apply for a new one from gov.co.uk. Most buyers also require an MOT Certificate, and your service/handbook.