If the front end user navigation is improved and displayed the folder structures I would rate this product 5 stars!
Thanks for your feedback . In fact, the two mentioned features were in Documents Seller already :
1. For navigation, users can navigate easily between pages : Category page, documents list, checkout.... They can also come back to previous pages using the links in the pathway. Did you enable pathway module on your site ?
2. For folder structures : Documents seller allows you to place your documents in folder / sub-folders already.
BTW, Documents Seller is an on-going development extension, so suggestions from customer like you is always welcome :) .