So I actually had this thought in the back of my mind and realized that this doesn’t actually exist in the custom server settings.
Basically how it works: You can sell a specific item in the list and you can remove or re-add those items. You can also sell a limited amount or unlimited amount like how golden apples are limited to 10, now you can make that infinite. You can also sell it for a specific amount of resources which you can set, and also set how much you can get from that. Example: A stack of wool costs 10 gold.
But in general this is my suggestion really, so do whatever you want with it.