I guess that could be done as part of a gallery -- but you could also do it by putting a 'sold' watermark on the original and re-uploading the image?
But uploading is all manual and stuff. I think we got Woo Commerce to automate it ("sold" was either on or next to the thumbnail). Still, the icon itself wasn't as important as preventing others from purchasing a license right after someone else.
Anyway, I decided a while ago that I don't want to run my own site right now, so it doesn't matter that much to me. I'm just throwing the idea out there because of my experience in the self-publishing world.
Premade book cover designers only sell a design once. There is also a demand among authors for exclusive photos and art that doesn't cost thousands of dollars. I know one
site that offers exclusive stock photos for $300 each, but they work though email, which is what a lot of the premade book cover designers do too. There might be a niche in automating that process, but it could be too tiny.