I like this plugin, it's really well made! I'm just trying to figure out how to let normal players (non admins) use admin shops. I don't want them to be able to create admin shops. I don't mind the fact that admin shops have anunlimited supply, since it's a low populated server.

I can't seem to find the permission for it. I'm using LuckPerms and other permission stuff works fine for me. Can you help me in the right direction?

You can find all permissions with their default values in the wiki: https://github.com/Shopkeepers/Shopkeepers-Wiki/wiki/Permissions

Players should be able to trade with admin shopkeepers by default. So maybe check your permissions setup to verify that your players have the 'shopkeeper.trade' permission.