How to execute a Sell gui for items in a players inventory?

So I have been able to make a shop where i sell items, the items get send to the inventory which i made a datasave for. Meaning whenever a player joins his data, inventory is the same. The matter i havent been able to fix yet is how do i make a selling feature of certain items inside the inventory?

My thoughts:

I should start with getting the inventory, meaning all the data, items saved in it. I should select or move the items to a different inventory the selling one. Everything inside the selling inventory should be removed directly from the inventory itself which should remove it from the datasave. meaning i need to fire an action to make it save de inventory without the items otherwise easy duplication.

Items now must contain a worth and coins proportie. Coins for the buying value and worth for the selling value.

I havent been able to find a proper guide on making said thing yet if someone could help me realising my thoughts, or send me a videolink would be great.

I am really lost in how to make it all function right now.