So should they sell their books in your Ecwid store? Please also clarify what you mean for expected earnings - it that some kind of statistics of how much they plan to earn?
The matter is that to get selling reports from Ecwid one needs to sell some goods in it first. There is no functionality that lets estimating future sales.
So if you would like to sell your customers books in your store as drop-shipper - this is possible, but you will need to create a custom script for that, as mentioned in the post I provided the link for. In this case you will be able to provide your customer (authors and publisher) with the sales info on their orders.
If you need some other functionality, please describe in detail how it should work.
As for the feature sponsorship and its cost, please contact the managers responsible at email@example.com