What is DMCA?  Simply, internet retailers and sites that host products, posts and distributes information gleaned from the internet must provide a way to have the information removed if it is infringed upon.  This process, if adhered to by the site keeps them safe from lawsuits for infringement.  In most cases reporting means filling out a form that asks you to identify the material infringing and provide verification you have a right to report it.  Again, as this buffers them from lawsuits, they are usually quick and efficient in removing the infringing material.  This not only applies to sites, it applies to hosting companies that "host" sites that infringe.  Meaning, if xyz retailer sells infringed goods, and you can not get them to stop, you may be able to file a DMCA takedown notice with their hosting company ie.. HostGator, GoDaddy, Ipage, Shopify etc.  You can also report any links that Google provides in search results, image results or imagery within Youtube and have that taken down.  Meaning, if xyz retailer shows up selling infringed goods in Google shopping and there's a photo on Google image of it, you can have those taken down too by reporting to Google.

Special note:  If you are dealing with a foreign company infringement, many countries do not comply with US copyright law but you can work fairly effectively in tamping down their US sales by reporting the http links of which your property appears on social media, search platforms or sites that are located in countries that do comply with US copyright.

I am not a lawyer and this information is being shared purely from my experiences as an artist that has been infringed upon often.

More on DMCA-

https://www.dmca.com/faq/What-is-DMCA


Etsy:
https://www.etsy.com/legal/ip/report

Amazon:
https://www.amazon.com/report/infringement

Pinterest:
https://www.pinterest.com/about/copyright/dmca-pin/

Google:
https://support.google.com/legal/troubleshooter/1114905?hl=en

Facebook:
https://www.facebook.com/help/contact/634636770043106