what we do if old owner died or unknown and never reply to his email although he sold the iphone and not in lost mode?

Well, regardless of the reason that you cannot contact the previous owner of the iPhone, you cannot do anything with the iPhone to activate it and use it. The Apple ID and password are security info required to unlock the phone. If there was a way to get around it then people would be doing it all the time and there would be no security for those who still own their phones but have had it stolen or lost it. The only thing the phone may be worth a few bucks in parts. A person needing parts from same model phone you have could be willing to pay about 20% of going rate for unlocked phone for just the parts alone.