It is impossible to know if the seller is telling the truth in this situation or not. Everybody makes mistakes, this seller just claims to have made a more costly one than usual.
StubHub is a marketplace, they can't investigate every time somebody claims to have made a mistake in listing their tickets.
IF the seller re-listed the exact same seats for a higher price once the original order was cancelled, I could see how StubHub could easily call out the fraudster and ban them from using their site... but even then the seller could just sign up using another credit card and a friend's address.