That’s the issue. You don’t have control of them, they often re-sell it or even print them by hundreds to sell them in Amazon/Etsy. And that “Buy once, own forever” is terrible for asset creators, because they’ll reuse that assets over and over again while we don’t earn a dime. But like @Roy_Wierer.Seda145 said, it’s impossible to track them down.
What FAB can do, is force them to use those products only inside UE5. As you know, UE5 is now a powerhouse where you can create, modify or enhance any assets just like Maya. You don’t need to export them… Devs can do it all inside UE5. That way Epic protects the Artists asset’s from being exploited.