This practice of blaming the customer for damage they did not do is not uncommon.....yet it is unfortunate. This is exactly why you must rent from an honest and reputable person....even if it costs you a little more (which it likely wouldnt)

Take "time dated" photos of every scratch and ding on the scooter prior to renting. Put the time of the rental on the contract and make the rentee initial the time. Now you have your proof damage was there prior to rental. I have never once had a problem using this method.