You can report people that steal your stuff on Redbubble, Etsy, and Merch by Amazon, but should you? My (possibly controversial) opinion on the matter and useful links below.
### Redbubble
Policy: https://help.redbubble.com/hc/articles/201579195
Designated Agent for complaints can be reached at: dmca@redbubble.com
### Etsy
https://www.etsy.com/legal/ip/report
### Merch by Amazon
https://www.amazon.com/report/infringement