For AI training? Or why would someone specifically pay attention to watermarks? I believe there are curated datasets out there. And watermark detectors to weed out watermarked pictures from a pile of data. I don't think the general public sorts by presence of watermarks...
And I'm not sure about games and webcomics. They all have some logo somewhere, because they've been made by someone. If you don't want that, you're looking for white-label products. I think in the realm of privacy, and those product types, that's a small to non-existent niche.