I live in Vietnam and Samsung makes their phones here. I see mobile store buying parts (still the exact parts from official phones) from unofficial sources, put it together and copy the official imei of an official phone to cheat the customers. Then they put them in boxes with seal and papers look exactly like a new phone...Those Samsung phones (most popular is note2 s4 note3 s5) works like **** and often have China battery and other parts.
The stores often tell the customers that those phone is carried from abroad and sell it at a cheaper price. (I bought mine at Samsung showroom for 800$ and they sell theirs for 600$)
It's best to buy from certified retailers.
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