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.
When the reason given for the 'mistake' is obviously false, it's not impossible.
>they can't investigate every time somebody claims to have made a mistake in listing their tickets
They certainly can investigate every case of purchases in the $900+ range. You know, as part of their multiple hundreds of dollars in fees.
The problem is that you could not concretely determine that to be the case. Thousands if not millions of ticket sales are canceled for a variety of reasons on StubHub every single day.
They would be going broke trying to investigate all of them for fraud, not to mention open themselves up to potential lawsuits by the alleged perpetrators they wrongly defamed if they made a mistake.
They can't exactly force that either, but they should be able to force penalties or restitution if they fail to perform.