I think we spok about that a few ago.
that requires a deep integration. Is not just to use a line of code in the theme or a themplate. Besides, you would have to do the configuration book by book… or again more and more integration. Is a lot of work.
The best idea is to create a second web site and to do your own marketplace, a amazon style web site, so your users could offer the book there.
PB is a multisite, where each book is an independent site. To do an ecommerce administrators of the independent books is complicate as you have to do the settings book by book. Just few options can be share across all the books, not all the plugins works sharing settings accros the multisite. That means time coding.
Simple and easy. And later they could offer the link from the book.
In my opinion is a simple and fast way of start, so in the future, you can update the site with a deep integration, but to start like that, maybe is not really a good idea.
I have the same problem than you, i´m asking the developers of the plugins I need for such purpose to create a sync settings for the multisite. Some times they do a first step. But is slow as they have other priorities. Maybe in 1-2 years that is much simple, now is complicate without speding time and money creating such integration.
My sugestion, start to ask to easydigitaldownload you have that problem, I will ask too following your question (let me know where you write the sugestion) and maybe soon they offer a solution if they see there is a need in the community.