Determining the authenticity of an iPhone 13

Can we assume you are not the original buyer of the phone? You can enter your serial number here, which will tell you if it is authentic, but if you aren't the original buyer, there's no telling what someone before you may have replaced on the phone. Use this link to enter your serial number --> Find information about your warranty or AppleCare plan - Apple Support

You take a risk when you purchase a used iPhone.

Whether a used device is authentic is only one of the things which can go wrong for an unsuspecting purchaser.

See this user tip for the many other “trip hazards” at-play when considering the purchase of a used device.

The All Too Common SAD Reality of Buying … - Apple Community