Continue selling when out of stock allows customers to buy products even when there's no inventory. This means they are backordering a product. Now for Special order products this needs to be turned on, as most special order products don't have inventory available. However for regular stock products you can choose whether or not you want to allow backorders by turning this setting on or off.
1. Click "Content"
2. Click the "Set 'Continue selling...' for regular stock products" field.
3. Click "Save"
