Last year I sold a car on gumtree on behalf of my employer. It was a Ford Galaxy, every panel was dented, the interior was graffitied and generally very grubby.Otherwise mechanically good.(Apart from the tyres!) I put all this in the advert but was still surprised at the ammount of calls I got. It sold within a couple of days.
As gb155 sugests I got a signature on the V5C and also Name, address and signature on a receipt just in case any parking tickets came our way after the sale.