Never mind. I just solved my problem in the previous post.
1. Shops start out empty with no item-types in them.
2. A trigger adds every item-type that they are supposed to have at Map Init with current/max stock whatever you want it to be.
3. Another trigger keeps track of the current stock for every item.
4. When a unit is supposed to buy an item through triggers, you just fake it. Create an item in it's inventory and subtract cost etc.
5. To reduce the current stock of that particular item-type by 1, you just remove that item-type, then add it again with current stock = current stock - 1 (you get the integer that the trigger in point 3 worked on) and max stock = what it was before.
When you get to current stock = 0, this will be displayed correctly in the shop.
So.. if you haven't solved it since you last posted, this is a solution that should work for the both of us