What is this for?
After used products have been sold online, there is not much reason to keep the listing up, even if it shows out of stock. Used products are generally going to be 1-off products, usually from in-store inventory, with unique descriptions, images, and levels of wear and tear. This option allows you to hide the product after it has been sold.
Using this Functionality
To use this functionality, create a product category (Products > Taxonomy > Categories) with the code HIDE_WHEN_SOLD_OUT. Then add that category to the used products you want to have hidden after they are sold. Once they are out of stock, a background process will mark them "internal use only" so they will not be visible to customers. This process happens overnight.
1. The background process will mark products "internal use only" but it will NOT remove "internal use only" from products in that category that are in stock. This is because the product could have been marked "internal use only" for any number of reasons. If you have a product like this that you want to make available again, you will need to uncheck "internal use only" after adding stock.
2. The background process does not check inventory for waiting orders, since an order could be canceled until it is shipped. The product will only be marked "internal use only" after the product has been deducted from inventory (that is, shipped, or inventory adjusted to zero from coreSTORE).