Making an item unavailable for purchase from a shop

[Spacebuns]

I want to make a Goblin Merchant able to sell a certain item only once, after selling it will be unavailable for the rest of the game, without affecting any other item in stock. There's a "Neutral Building - Make Item Unavailable From Shop" trigger, however it only works on a shop that has the "Sell Items" ability, but the Goblin Merchant has no such ability by default (it has Shop Purchasing Item, Select Hero, Invulnerable).

 

[millzy]

why cant u add "Sell Items" ability to the goblin merchant?

 

[Chaosy]

There is a GUI action for neutral buildings that can remove/add items I think.

 

[Spacebuns]

I found the solution.
Turns out I have to give the "Sell Items" ability to the Goblin Merchant and then, via triggers add the item to shop and remove it after someone buys it. It looks like "Sell Items" is some sort of dummy ability that's required on a neutral shop for neutral shop-related triggers to work.
Items in the field "Techtree - Items Sold" cannot be removed by any means.
Warcraft 3 is weird.