Imagine that someone takes your work and puts it on the marketplace without KYC. How could you accuse this person?
I would contact the marketplace, present my proof and leave it up to them to take down the seller. If it was posted at another art site as being created by a different person - same thing: present my proof and leave it up to the site to take it down.
If a marketplace or website does not comply - bring lawyers into play against the sites.
So you didn't look more closely at a stock image, without KYC and with payments in Bitcoin?