The problem exists where only 100 items lets say of an item are produced in total. The customers are aware the item is a rare item.
The are 900 customers registered to purchase the item using stock notification module.
Now given this information 800 wont be able to purchase this item, the logic of purchase means that whomever opens their email first or gets to their email first is able to purchase the item, which means constantly checking depending on time zones etc, unless something is announced of a specific on-sale time.
There is a method that Nike use via their Nike Snkrs app. The same problem exists for their Limited sneaker releases. They have ability to enter a raffle but selecting the Enter button, it will then randomly select a number of users who have pre-entered their payment information and the lucky selected will update to "Selected" and then these will essentially become orders. Those not selected will be notified they were not selected this time.
This seems like the fairest method in order that a random amount of customers could be selected based on the criteria who entered only once.