Generally it’s best to spend time improving items than chasing copyright.
Update the item graphics to include a version number or “new feature” banner. People searching for the item will find Envato first and see the legit copy comes with more.
Track down the buyer and notify Envato. Get their IP address even. Envato might be able to find the buyer based on that and disable their account. Or buy it from them and ask for a license code to access support, get the code, send it to Envato and have it disabled.
Another option is to add a license code check into the theme. Restrict usage based on that.
Funny story someone was reselling Envato products cheap. Buy it once, hope to sell it multiple times. They had directory index on hosting, could see what items they bought. Someone just bought all their own items through the pirate, then got their hosting shut down. So this guy made a profit off the pirate. Lol