I set up Google Alerts with the titles of my designs to be notified when they are put on sale illegally. For example, one design that gets stolen a lot is “Overthinking and also hungry”. When thieves try to sell t-shirts with that designs, they don’t even change the title so I created an alert for phrase “Overthinking and also hungry”.
This way, I get Google Alerts as soon as Google finds out about the infringing listings. And as soon as I get notified, I can send a DMCA takedown notice so that it’s taken down immediately.