I think decentralizing Amazon is more possible than you think.

The main first step would be breaking apart functions. Amazon controls posting, search and rank, storage, and delivery. You already have many sellers, multiple storage locations, and multiple shipping companies. If posting listings and ordering the search results was out of Amazon’s control, and Amazon couldn’t wield their power to compel sellers to store at their locations or use their shipping services for a huge fee then we’d already be most of the way there.