How to create a complete shop ?

Hello,

I'm trying to create a shop in my game that uses custom currency displayed in a GUI. I'd like the shop to open when pressing "V" and close when pressing "V" again or "Escape". The shop should have multiple categories (e.g., blocks, ores), allow me to buy or sell a custom amount of items, and display my money in the corner of the shop.

I'm not very familiar with variables and procedures, so I need some help. Specifically, I want the shop to function similarly to shop plugins found on servers.

Thank you for any assistance you can provide!