I am sure something like this has been suggested before, but hear me out:
Developers sign up and either commit a portion of their sales and/or pay a monthly fee,
This grants them a membership in the Co-op (which includes a profit share),
Developer (members) have access to the source code of all the software in the stores.
Consumers/Users create an account and are now able to search, browse and purchase precompiled binaries / one-click installers of the various Free Software programs available across all the stores,
Users can request developer access to get to the source code of any programs that interest them,
Registered users could potentially have an option to compile the software from sources themselves without having to purchase the program.
It is decentralised and federated, meaning that anyone can self-host a marketplace, and anyone who signs up at one of these can search, browse and purchase software from all the other marketplaces.
Each marketplace is its own co-op (not suggesting that all the devs from these stores are all members in one giant co-op, unless that is actually a better way to go), however the various marketplaces could themselves in turn be members in a larger co-operative organisation, although you would want to avoid the potential for the centralisation of control and power.
Only software which conforms to The Four Freedoms is allowed (not sure how to do this in a federated space, but perhaps users and developers can report software that they feel violates any of these, which could then be examined or audited).
Would probably require some innovative thinking around things like: “Federated Identity”, different national currencies (and payment processing) or digital currencies (the ideal would be to trade without involving banks at all) and possibly even a solution for Federated GIT or some form of federated version control & development (so that devs from different marketplaces can share and collaborate on code).
Just a thought.