As an op, I created a shop but normal player cannot trade/buy/sell stuffs with me. If I deop myself, I also don't have the permission to use the command of creating shopkeeper. How can I let other players have the permission to trade with me? Do I need to install other plugins?

Bukkit v1.12.2

Your players should have the trade permissions, as well as the permission to create player shops, by default. If you need finer control over permissions, you can use a permissions plugin. But trading and player shop creation should work by default already, without any permission plugin.

Is this a spigot 1.12.2 server? Are you using any plugins which might interfere (permissions plugin, plugin manager plugins, ..)? Would you mind setting up a fresh spigot server with only shopkeepers running (no other plugins) and try to reproduce your issue there?

Just checked to be sure, and permissions work for me by default. So it has to be something related to your setup.