Apparently, you have some doubts or you would not be asking the question here.
If you don't trust the reseller, return it. Apple does not permit demo units to be sold. They do not have standard software on them. Return it and go buy a phone from a legitimate, trustworthy reseller. No authorized Apple reseller would willingly sell a not-for-resale demo unit.
Return it. That phone should not have been sold. It will NOT function like a 'normal' phone, and it will not have a warranty at all.
But seller said ,get the premition for sell.(apple).
Functions are ok.
Demo phone is suitable or not
