DarkRP NPC Shop Help

Hello. So I have a script that spawns an NPC, and I made it to where when used, it opens a simple derma menu. Nothing fancy. What I want know is how to add items for the NPC shop. The Gmod NPC_Shop Tutorial did not cover that part…in fact the tutorial is kind of broken…but thats not the point, the point is, can someone please tell me the code for adding a buy-able item from the NPC.(If possible, add a way to make a custom check for a VIP only NPC Merchant.) Please, and thanks to all!