You are shopping in Hong Kong and find a video camera advertised for an amazing price.
You decide to buy it and, as the transaction goes through, the seller tells you they are out of stock and the camera you just played with is just for display, and doesn’t have a case or charger. “No worries, he says, we have one across the street, at the other location of our store. Let me call them, to bring us one.”
As you are patiently waiting for a long time, he will mention the camera is not really that good and some people had a few complaints about it - “but nothing major to worry, my friend”. What he is trying to do is get you to buy an upgraded version, which is more expensive.
Plenty of tourists fall for that, which is why the scam works for the seller.
How to avoid: this one is quite simple. Pay only when you have the product on the counter, in front of you.