Earlier quoted context omitted.
Did the promise not to cancel orders that they received the payment for in the future?
How could they possibly make that promise? They don't own the tickets, and they're not really selling them. They're facilitating a transaction between two parties.
They could also add automatic sanity checking on listings, so sellers cannot accidentally list tickets at too low a price; and a manual offer acceptance stage in case the seller has chosen to list on multiple marketplaces.
They could also require that when a seller is selling a ticket, that the seller proves they actually have the ticket. If stubhub want to facilitate sellers engaging in speculation by selling tickets they don't own, they could clearly mark such listings, with the seller placing a security deposit so their obligations can be met even if the market moves substantially against them.
In the event a seller works around these protections (such as by submitting fake proof they have the ticket, or refusing to meet their obligations if the market price of a ticket they shorted exceeds their deposit) stubhub could take care of forcing the seller to meet their obligations.