they aren't entirely stupid when people register with the same name and address and possibly even bank account, that just changing your email address makes you a "new person"
unfortunately its all part of the terms and conditions you signed up to when you use Paypal, they don't hide it, the easiest thing is to just not accept Paypal payments and do cash on collection only - you can also do this via ebay
as others have said, you agreed to paypals terms and conditions, paypal don't have to prove you weren't scammed, they only have to prove they acted within their terms and conditions and that you owe them money, its up to you to go after the scammer