Markets were originally decentralized, and while that has its problems, a decentralized market is miles better than a monopolized market.
Like, are you thinking of Etsy or Amazon or something? Because those are all run by a single point-of-sales and logistics collectives.
What we’re talking about is basically building a means for getting all the websites around the web of small shops and such (or in this case all the various game store fronts like steam, itch.io, GOG, and EPIC GAMES) and giving you client which allows you to browse and order from them simultaneously. All that store’d have to do is add the protocol to their server and add themselves to a list.
Each server would likely have to utilize a payment service. In that fashion it’d be no different than how stores host their own websites you can order from. In my mind, the federated protocol would simply be a means for a person to browse stores similar to how one can navigate a mall or market.
For games, the further benefit after would be that via a client of the protocol, you could then download your games from the various stores in a singular library page.