There is no way that you can prove that it wasn't you who had the repair done.
If you buy the car, it doesn't matter; if not, it will reduce their buy-back price and may mean that you will have to pay. Many people with PCP deals decide to buy the car when they find out how much all the dents and scrapes are going to cost them. A private buyer will be nowhere as near as good as them at spotting blemishes, and you would be under no obligation to point them out so long as you don't say there aren't any.