It's actually quite easy to know where legitimately sold product goes since most companies track serial numbers. It also isn't unusual for supply agreement to have restrictions on the ability of a customer to resell product. In a case where a company purchases components and secures the right to resell, those resale clauses usually would have a right of first refusal for the manufacturer to repurchase. Most companies know exactly which product they sold to which customer or distributor, so if the parts end up in devices that ultimately end up coming in for a warranty claim YiHi would quickly figure out where they came from.
It's more likely that any product that gets used in unauthorized devices is either stolen or counterfeit. I used to work for a very large analog semi company negotiating their supply agreements and we often had company's seeking warranty claims on stolen or counterfeit product. We had armed guards accompany shipments from our Fab to the shipping point since the trucks were subject to hijacking attempts. It also happened that we had problems with shifty employees taking product and selling it out the back door.
That's interesting. So it actually wouldn't be that hard for companies like Sigelei to find out who is selling their products to fasttech. Why then hasn't this supply chain been shut down. I do know if you buy from Fasttech, the manufacturer’s serial numbers are usually blacked-out on the warranty cards. So all warranty issues must be handled by FastTech. So either Fasttech just takes the loss or returns it to their supplier for replacement.