It’s not clear to me how this idea, a decentralised place where you can access a lot of different stores, isn’t just the web. There’s a lot of mature alternatives for running web based ecommerce storefronts.
The value of Amazon and other marketplace systems, what I think many would say is their distinguishing factor, is capital L Logistics. It’s the warehouses, the return systems, quality control, customer support, international shipping and importing, inter-warehouse trucking and a network of last mile delivery, all tuned to minimise costs. An open source platform that handles all of that somehow would actually be revolutionary, but even the design of the system would require actual years of user research and mastery of systems that are truly arcane.