How do i know iphone 4,4s,5 has registered with apple and factory warrenty?  Does iphone has any special note can make me believe to buy? (not fake)

I have few seller want to sell new iphone 5 to me with the good price but they said no warenty, i want to buy but i worry have the problem, what are special note on iphone 5 can make me trust?

iPhone

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