example for countable potions
if u buy an item , then the trigger would activate, so it would check if the buying unit alredy got an potion. if so, it would increase the counts of it (=potion) and skip the rest. but because he bought another potion he would have a new one in his inventory (one with 2 counts, anothe single one). so he needs to remove the sold item when the trigger counted the old one.
but i guess, this is only when u actually use an event like "a unit bought an item". so ur post is all right. ^^