[karzeem]

They could charge sellers the greater of the purchase price or the market price of the ticket at the time of cancellation.

[mbcrower]

They could, but to business like StubHub each seller is probably more precious than each buyer. I'm not saying it's fair, but it's a lot easier for them to find new buyers than sellers.

[DrScump]

... assuming that the seller didn't abscond with the payment and cancel the credit card used to guarantee the transaction long ago.