Purchased Oliver Cabell Shoes and stated it was in...

Purchased Oliver Cabell Shoes and stated it was in stock. Next thing you know, it said it was out of stock and therefore was refunded a lesser amount due to the exchange rate which is based on the date of the refund instead of date of purchase which I understand. However what I don't understand and particularly frustrated about is why did the online sales system allow me to make a purchase if there wasn't any stock? I lost about 16 dollars in the currency exchange due to this inconvenience.