When you buy things on the internet, you can't always be certain about the quality. Is it genuine or just a bad copy from far away?
OK, you make the purchase.
You'll have to try it on, especially if it is some clothing. But what about diamonds? How to determine if it's genuine?
You will have to ask an expert. Maybe it is obvious to consider whom not to ask but think again. Who do you ask and why? Can you trust their credibility, or is that fake? Think twice.