OK, so there are building permits out there... why not have a commercial permit and allow people to turn houses into bar/restaurants, general stores, or specialty stores. They can set prices (which would rarely be lower than vendor prices) and give access to the wider community to rare items.
Logistically, you could just have a cupboard that holds 20-some items that customers and shop keepers could both interact with, and then you have those items locked in the cupboard until a cash exchange happens. Then the shopkeeper gets a menu option when you click on an item in the cupboard that also has "give to" and then you give it to someone else in the house. Or there's an option "sell to," you pick a person in the house, and then you put in a price offer and they accept or not.
Mostly this would be fun for someone or some group of people to run restaurants/bars where people can go and mingle.