Really? I have been buying from yard sales for years and I agree for the most part but when it comes to certain items I expect the seller to be telling me the truth and if I paid good money for something I expect it to work.
I have been looking at camera's on craigslist and this has made me think twice about it. I would not expect to drop $200 plus on a camera and be told it works and get it home and find out it didn't. I would expect my money back.
All that said it does depend on the circumstances, in this case the buyers saw this
scooter working and now says it doesn't. A case could be made that the charger is not working and it will not charge(giving them the benefit of the doubt) or they could have done what everyone thinks they did and strip parts from it they need or even bring back their scooter and try passing it off as the one they bought.
Not sure what I would do in this situation but next time I would make sure I marked the screws or something so you can tell if the item had been tampered with. Even mark the item somewhere so you know whether or not it is even your item.
This is a difficult call.
Good Luck I feel bad for you being in this situation.