Server’s offline to back it up and update some things.
How about, instead of player-owned shops, we have a global shop that players can buy and sell to. But the inventory of the shop is entirely dependent on what players sell to it.
So, if the shop started with 0 items, no one could buy from it.
Then someone sells a cookie to it. It now has an inventory of 1 cookie.
Anyone could buy that one cookie, but that’s all they can buy is that 1 cookie. (Unless someone sold more cookies to it.)