If you are buying online you'd need to pick the shoes which you know [from their measurements] will fit you. If you accidentally choose the wrong size can you demand for a refund of your money if the mistake is yours? Or should you negotiate with the seller, send back the shoes and pay for shipping both ways? Buying online can be complicated
It really depends on where you're buying from. If it's from Ebay from a personal seller, they usually have a return policy that clearly says if you can return it. From most retailers (like if you buy directly from the store/company), they almost always allow returns within a certain time frame.
Most likely you can return them for an exchange (maybe a refund). You will need to pay to ship them back and depending on the seller/store you might need to pay to have the new pair shipped to you.