This is very interesting to me and I've played with this kinda idea a few weeks ago, the Activity Pub proposal you linked seems very sensible for communicating between actors but doesn't really offer much of a path to create a platform. In my view creating a platforms is the reason this should exist, because current platforms (Amazon,Ebay,Uber, AirBnB,DoorDash,Lieferheld) are mostly just engaging in rent seeking from buyers/sellers on their platforms. Rentier Capitalism

I don't believe a protocol can sufficiently challenge the current players without an underpinning organizational structure that ensures fairness and transparency to both sellers and buyers, when it comes to moderation, indexing, and categorization. Especially moderation but also hosting will have costs, and the consequences for bad moderation are likely much larger with commerce than with social media. So I would like a Coop with significant control from both sellers and buyers to provide the public facing platform which then federates with the Stores which can be self hosted by sellers (potentially as an extension to existing eCommerce Software).

Or alternatively two Coops if it's not reasonable for the sellers to host their own Stores e.g.: Uber and AirBnB, here the sellers should outright own the one providing the Stores, and own the minority in the one providing the Coop. Obviously middle grounds could also exist where e.g.: a Platform for Delivery food federates with seller servers that are hosted on a local level by Coops comprising of restaurants of a region.

I very deeply believe something like this could make our commerce much better and fairer, and while getting it of the ground might be hard, I think because the sellers make money on these Platforms it should give real incentive to develop both the tech and the legal orgs as well as advertise for them, and for the sellers to invest real money into it, or maybe agree to kick 1-2% of a purchase back to the coop.