This is not really a scam but an idea to someone that is used to travel in this kind of countries. When you travel in Vietnam you will see a lot of nice bags and another kind of items. If you see something you really like just ask the price and leave. Walk a little bit more and usually you will the exact same thing again. Ask the price once again and walk away. When you walk away sometimes the shop owner will make you an instant discount. If you do it 3 different times you will have a good idea what price you can start to negotiate. This will save you a lot of money if you pretend to buy a lot of stuff.