I have no idea how they think they can enforce it, period.
I own a brick and mortar store. Let's call it brattle grounds. I own an online store. Let's call it warbitz
battle grounds buys stuff wholesale. Sells some. whatever.
Warbitz does it's thing. Sells bitz. Shopping cart, 20% off, the whole deal. GW hear's about it. Asks me to stop. Probably even gets nasty, sends cease and desist letters, maybe claims copyright infringement or some bullshit. (all of which is completely meaningless and not true).
Warbitz tells them to go pound sand.
It's completely legal. It's called the right of first sale. The only thing that GW has over brattle grounds is it's contract with BG (totally different BG, here)....which is mostly an agreement to sell at wholesale. The main penalty of which is forfeiting being provided goods at wholesale. Except GW doesn't know Warbitz and BG are related. They would probably demand warbitz tell them where they got the goods.
No. None of your freaking business.
Now, GW could probably find out that the two companies are related with a little detective work. Probably not even super hard to figure out. But this is not, generally, something that companies do. And the man closest to the scene is the GW sales rep......who frankly just wants to sell product to brattle grounds.
Just saying. It's not something I see as really enforceable in any meaningful way. Especially since the base economic activity: buying it, and then reselling it in whatever form you want.....is perfectly legal.