Buy In Advance Question

The out of stock actions have a question mark next to them which says the following:

[attachment=13280:buy in advance.JPG]

So buy in advance requires that the stock value be positive. Surely this defeats the purpose? Shouldn't it allow customers to buy the product specifically when item isn't in stock? Isn't that what buy in advance means?

Am I missing something really obvious!? :confused:

Basically I'd like to allow customers to buy products that are out of stock, but I'd like them to know it's out of stock and that they are placing the product on back order.

buy in advance.JPG

Here we can see in the Buy in Advance section

The product’s quantity in stock can also become negative, even if negative inventory is not allowed on the “Settings → General” page.

Not sure how it works