what to do if someone sells my Themeforest item in other marketplaces?
The DMCA law states only the “copyright owner” or an authorized agent on their behalf can submit a DMCA notice. You own the copyright, so you must enforce it yourself.
If it’s not being sold on envato then there is not much Envato can do - only the copyright owner can submit a DMCA or claim against this type of issue.
First, make sure to send a DMCA takedown notice to the website or its host. Instructions are here: How to send a DMCA Takedown Notice (just 4 steps!)