How is a service like Shopify, where any non-technical person can open a store and sell things not an open marketplace? You don't buy through Shopify anymore than you're buying through WordPress if the seller chooses WordPress for their company website. The web is the open marketplace. Shopify isn't Amazon.

Also, I don't hate the idea of what you're building but keep in mind lots of people selling stuff on services like Shopify have zero technical skills and most people who shop online are also normies who will know nothing about open models and whatnot.