I’m not sure they’d want the legal hassle.

As long as Steam allows skins trading these sites will exist. I can’t see them removing this feature from their community because of activity off their platform.

Locking a trading account and nuking the inventory just means that one site will shut down - the operator will likely just set up a new one and a while bunch of users will be angry at Valve.

If enough money is at stake Valve might even find themselves sued by the site operators. “Tortious Interference” is what it’s called here.

If consenting adults enter into agreements outside of Steam, what business is it of Valve’s to interfere?