There are no standard shoe sizes in North America. Each manufacturer makes his own decisions about what measurements he will use for each size.
Your best bet is to ask a seller before buying for the length of the shoe and the width at the ball. In centimeters, or even millimeters, for accuracy.
In this case, explain to the seller that the shoes are unwearable, and that you want to return them. No recriminations. She gave the size she saw and you did not ask for further measurements.
You will need to return the shoes with Delivery Confirmation.
If the seller is in Europe, this could get pricey since ExpressPost International starts around $40.
You may find your best bet is to accept that there was an error and resell the shoes on eBay--- with very exact measurements.