^Ah, I've misunderstood your question previously. I thought selling/giving an item to a shop and creating or removing the items from the inventory slot of the unit. As super good mentioned earlier, I don't think you can manipulate or change the item stocks that can be sold by the shops or item sellers through Trigger Editor as they were only initiated at the Object Editor only, in my opinion. But there is a possible way of doing what you wanted to... You can replace the unit seller by the another unit seller that is having different item stocks through Object Editor and by triggering the replacement. The method is untested and I'm not sure if it works but you should try.