Every store owner knows that sometimes it happens that we run out of stock of the items. Some items we can get back to stock fast, and some not.

The ones we get fastback in stock we may want to be able to sell (pre-order) for our customers. But for some items we as a seller don't know how fast we will get them back on stock.

So we simply can't leave them on sale, because there is a possibility that these items can be never on stock again, due to any reason may it be bad supplier, long delays etc...

By default opencart allows us to sell out of stock items, but if this feature enabled ALL of your out of stock items will be available to buy (the ones reached 0) without any control and second option by default you can choose from is to disable ALL items the ones is out of stock, leaving you with no extra profit you could have selling these quickly back in stock items.
What leaves you as admin with a profit loss for items the ones can be soled but yet not in stock.

So here comes this module where you can define based by Out of Stock status the items to be allowed to be bought or no. Also you can define which customer groups can buy and which ones not.

This module allows you to set Out Of Stock statuses the ones will allow to buy these items and the ones not.
All you need to do is simply select the Out of stock status you want to allow purchases and set a user group to be allowed for purchase the items once it reaches 0.

And you are done. Customers will be able to buy these items you set to allow to buy, even if they are out of stock. When other items (with other out of stock status) will not be allowed to purchase.

This is a must to have modifications for store owners who do not want to loose their sales even if item is out of stock but for really short periods. And those who don't want to hide out of stock products on their websites (due to google indexing).

Now you have full control of your items which ones reaches zero quantity.
PS: you can use this module even for UP SELL method.