As delivery store you mean? Who's gonna get the credits? The owner of the FC? Shared among the group selling?
I wondered if it would be that someone has an FC, and people shop at that FC because its either somewhere really convenient or that it serves as a hub. My thought is an FC is like a mobile clubhouse- a PMF can take control of stations but never benefit from the infrastructures markets. FCs allow that, so done right they are self sustaining and the larger the squadron / buyers / sellers the easier they are to run.