My favorite thing in the world is when specs include language like "anything else I might not have thought about!"
Haha.
Ok so anyway, that’s helpful but there’s definitely some questions you need to answer. When an author requests payment, is that automated or no? (It says it’s just an email, but how are the payments recorded in the back end? Etc.) When an author uploads a book, is there an approval process or are they published live immediately?
In any case, this would certainly be possible using MODx + FoxyBack + FoxyCart, or possibly Kiwee or TC, but I don’t know as much about those. You’d likely need to map our all the functionality for the authors, and tie in the self-hosted downloadable FC code into FoxyBack, but it’d be possible.
X-Cart’s Mall version might work as well, though. Worth looking at, even though I’d hate to use anything other than MODx (personally).