Ok, this tutorial is for having an NPC holding an unlimited ammount of one (or more) item(s)
You will need one variable
Lets say there is a box that can hold all your potions for you
It should look like this
Text: I could store potions in here (this is not necessary, but what the heck)
Text: Put a potion in?
Show Choices:Yes, No
When [Yes]
Conditional Branch: [Potion] in inventory
Change Items: [Potion], - 1
Control Variables: [Potion Ammount] +=1
Else
Text: I don't have any!
Branch End
When [No]
Text: Want to take one out?
Show Choices: Yes, No
When [Yes]
Conditional Branch: Variable: [Potion Ammount] >=1
Change Items: [Potion], +1
Control Variables: [Potion Ammount] -=1
Else
Text: There's none in there!
Branch End
When [No]
Text: Then what am I doing here?
Branch End
If you want to be able to put multiple items in there, add another choice between those items in the beginning, for money, you could have the player put in like 10 gold at a time rather than one. It would take forever