easy to federate: announcements reasonably practical to federate: reviews difficult to federate: payments, user reputation
One of the things Etsy offers to sellers is fraud detection. Not just stolen cards, also people who lie about not receiving products. Centralized services function as a "root of trust"; it's much harder to create a trustless system for this.
It's what (yuck!) blockchain algorithms were invented to solve, but they have their own problems.