It will let the customer purchase into zero and negative amounts without indicating that any purchase of an item when the inventory goes from zero or below will be a backorder. Basically, if a product has an inventory of 1 it will allow the customer to purchase 4, in a single order, without indicating that 3 will be a backorder. The customer is left unaware that 3 items are not in stock, and we the merchants are stuck dealing with this. They require you to check "continue selling when out of stock" to enable the backorder function but did not take into account going zero or below in inventory on a single purchase. Not to mention Google lists your products in stock when inventory is zero or below because of the "continue selling when out of stock" requirement to use the backorder function.

"Your products are marked as unavailable for the Buy Button channel." "Error enabling availability. Try again" Here is a direct quote from the two week Shopify investigation into a full lost week of sales in the lead up to Christmas (aka the best time for ecommerce in the year): "The issue you encountered with your store was related to the Early Bird app. As previously shared, based on our logs, this is the only app that was installed and modified your product. This app changed the subscription settings for your product, which affected its availability across different sales channels, including the Buy Button channel." THIS APP IS MALWARE - STAY AWAY FROM IT ONE DEFINITION OF RANSOMWARE: "a type of malware that permanently blocks access to the victim's personal data unless a "ransom" is paid." Didn't work as required, changed all my product settings (without me knowing) and when I uninstalled it found a way to make the settings permanent and cost me way more in sales by disabling customer checkout and payments than it promised to make unless I signed up to their subscription even though it doesn't bloody work.