Subject says for itself.
The way I see it:
(At first we need to add couple of buildings inside or outside each city.)
Leader of faction speaks with some NPC, this NPC offers e.g. tent, small building and large building.
Tent
Just a tent , like Buster has.
- costs 100 000 caps + 5 000 caps weekly (rent + shopkeeper salary)
- space for 2 guards
- 2 locked crates - 1 for cash and 1 for items
Small building
Just like any bank building
- costs 200 000 caps + 10 000 caps weekly
- space for 4 guards
- has locked vault room for stuff
Large building
- costs 350 000 caps + 20 000 caps weekly
- space for 6 guards
- has locked vault room for stuff
Building cost & rent should vary from town to town (should be very cheap in unprotected towns)
Then leader chooses which guards to hire (also "no guards, please" is an option too). Guards occupy all guard slots and you should pay for them too(if you pay 500 caps per week for guards in tent, you should pay 1000 caps for same guards in small building due to the number of guards ). Ofc guards should have a lot of hp and should be sg/ew snipers (no exploding or bursting weapon).
The leader of a faction(or one of the important members) must pay weekly (rent & guards' salary should come in one bill) , otherwise shop will be closed after 1 day waiting period.
After paying the bill(to the NPC, which sold you a building), the NPC should offer to change guards (to cheaper or better ones).
Trading process
In each shop there shuold be 1 container for stuff. Faction members can put here anything, that would be immediately transported to the shopkeeper for sale.
From each transaction shopkeeper "puts" half of money income to the vault.
Also there probably should be some methods of changing shopkeeper's trading policy (e.g. Leader of a faction can say: "don't buy big guns", which will set all BG buing price to 0, or "store all miniguns in the vault", which will force shopkeeper to "put" all miniguns in locked container, apart from caps) and price policy (e.g. "sell all small guns for 2\3 of price" or "buy all Rockets for 2x price" to get what you need and stimulate economics - players would be able to sell unnecessary ammunition for bigger price and buy necessary for lower)
Guards' policy
-holstered weapon in shop is allowed only for faction members.
-all detected thieves are shot(probably no corpse looting for this guards), no chasing if the thief leaves the shop.
-everybody, who tries to get in vault(except Leader of a faction or important members) are shot immediately.
-vault doors open when the leader asks shopkeeper to do so
To avoid abusing all NPCs in shop should be neutral or should belong to current town's faction.
And yes, the shop can be robbed, but the robbers should be able to neutralize all shop guards plus all town guards that will be attracted by shooting, also the robbers should have a good lockpicker as the higher rent - the safer are locks in the shop.
All prices are for illustrative purposes only.
Well, that's it.
P.S.: Sry for terrible English.
Note: Buildings are not literary sold, u just need to pay (100k, 200k etc.) 1 time to "open" the shop... If you stop paying rent - you'll have 1 day to take everything away, otherwise all your stuff will disappear and the building will become avialiable for any faction to buy.