Customer will not know if their documents has ben sold to other parties. If the stealer don't make it public, I mean he is not sell in public, no one will know. But usually, the stealer will sell in an underground forum and some of them will tell from where the document come from but other will keep it secret.

It is like you get an offers via SMS from many parties. They get the data from the marketing and they are sharing their customer data to others by selling it or give it. It is why we as a customer must be selective choosing the site where we want to KYC. That will prevent the data breach that may happen on that site.