Before you write this book, I would suggest doing some more research on ImageKind.com. The fact is, most of the artists on ImageKind.com aren't selling any artwork. In addition, they don't see any value in paying $100 / year for a "premium" membership, and many of the artists are opting for a free membership instead.
Therefore, ImageKind.com doesn't really have a good revenue model. Earning $10 - $15 per sale isn't going to cut it. They need their artists to pay $100 / year in order to stay afloat, and that isn't happening.
I would hate for you to write a book on a website that has an unhappy customer base.