The problem here is that getting one's money back doesn't feel like enough, because you're losing out on massive appreciation of the purchase between when you bought it and now.
This is one of the most difficult customer service issues to solve, because the cost to correct the situation in the customer's eyes has no upper bound.
So a big part of this problem is that StubHub is letting sellers cheat buyers out of massive appreciation by not giving them a reasonably deadline for delivery, nor penalizing the sellers for failing to perform.
The potential costs to correct the situation don't have to be an issue, because StubHub could conceivably push those costs onto the seller trying (and in this case apparently succeeding!) to cheat the system. They could write their seller contract to state that the seller is responsible for all costs borne by StubHub acquiring replacement tickets, and that would remove their incentive to pull stunts like this.
The old Stubhub (pre-eBay) made a BIG deal of denied-at-door situations, and local managers had broad discretion to source replacement tickets, even at higher cost.
Nowadays, they realize that doing so is not profitable in the short term.
Next thing you know, people are going to start offering a lifetime guarantee that's only good for the lifetime of the product! ;)
I don't need to reed any user agreement to know that. And Stubhub is clearly a marketplace. I would assume this site is just the same as eBay or Airbnb. I know that are some responsibilities of a marketplace, but I also know that I'm ultimately trusting the other transaction part (not the marketplace).
If I think this risk is too high, I don't use a marketplace (personally, I never bought anything through eBay or equivalents myself precisely because of this).