I doubt they can stop this entirely. They can probably prohibit resale on third party sites and - to your point - prohibit scalpers outside the stadium.Is this an anti-scalping rule for the area around the stadium?
How can they prevent someone from selling and transferring a ticket to someone they don't know rather than just transferring a ticket to someone in their group?
Can they rescind sold tickets if they see them on a third party site?
But if two people from my group are unable to make the trip and I committed those two tickets to someone here on CF and took a Venmo payment for it there is almost surely nothing they could do.
In essence, that’s what I’m already doing with my group of friends I bought tickets for.