Full Question
My last time in China, somebody offered me the iPhone 4. I accepted as it was offered for 100 USD. I got three, but they were not original and at least one was unable to work. My question is, how do you control these sellers that sell bad or non-original Apple products?

Answer
In the past days, the "Ask the Experts" service has received a wave of questions about how to buy Apple products in China. Many of these buyers have been the victims of scams like the one you describe.

You can refer to the question "Can you get genuine Apple products from China?" on this same page. It explains why you won't be able to find REAL Apple products in China at a price lower than back home. It also explains why you will be a target of scams if you try to buy these products. Tips and best practices are stated in that post too.