Purchase Items Once

I'll get straight to the point:

I made a map in which one objective is to save 12 Ancient Artifacts. Some of them are lying around, some are held by enemies, and the most are in doodads.

Example: In the beginning of the level, A corpse surrounded by enemies has an Ancient Artifact. The corpse is a Merchant that only sells the Ancient Artifact item.

Problem: I can "Retrieve" the Artifact, but then there are more.

Question: Is there any way to make it so that the item can only be bought once? Otherwise, the map makes is easier to cheat.

Thanks in advance.





-KillR

Catch the event that the item is bought and then remove the item from stock.

Simple, there's a trigger action that's called something like "Neutral Building-Remove Item-type (From MarketPlace)" or something. Just make it so that when they buy it, it removes the item from the shop. :D
Oh, and make sure that the merchant unit has the "Sell Item" ability, or else the action won't work on it. I'm saying that cuz I didn't do that and it took me forever to realize that that was the problem =P .

EDIT: Bah, AIAndy beat me to the punch. ;)

Why not have the item next to the corpse?

Or detect the item being picked up, then delete the corpse.

Alright, so I tried the trigger, but it still doesn't work. Here's what I had:

Artifact Recovery
Events
Unit - A unit Acquires an item
Conditions
(Sold Item) Equal to (Item carried by (Buying unit) of type Artifact)
Actions
Neutral Building - Remove Artifact from (Selling unit)

A few notes this time:

-There are 7 Units
-1 is a hero "Ghost Commander"
-6 of them are Ghosts (Starcrafty)
-Commander cannot wield the Artifact
-Ghosts are supposed to retireve the artifact by buying them from doodad shops (Shops with doodad models, like corpse and Ruins Obelisk)
-Some of the doodads contain more than one item

Thanks in advance for suggestions here.






-KillR