While buying a used iPhone, there are certain things you need to know. Case in point, you should check if the iPhone is new, refurbished, replacement or personalized by engraving just to ensure you know all the important things about the device you are going to buy.

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How to Find Out if iPhone is New, Refurbished or Personalized

>> Launch Settings app on your iOS device.

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>> Now, tap on General in iPhone Settings

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>> Tap on About in iPhone Settings

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>> Check out Model and then read the model identifier.

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It looks pretty much like MN572LL/A, and the first character helps you figure out whether the iOS device is personalized, refurbished or new.

M: It refers to the brand new device.
F: It means the refurbished device, which been through refurbishing process.
N: It means replacement device.
P: It refers to the personalized device with engraving. That means the iPhone was customized with an engraving on the purchase.

That's it.

Source
http://www.labisblog.com/2018/02/how-to-know-if-your-iphone-is-new.html?m=1