Whether from people you know or people don't know, for your own safety, you should stronglyfrowm against buying of second hand phone. Asides the chances of the phone being a stolen phone, you could end up incurring losses for yourself.
A friend of mine saw a phone and on Facebook, it was an infinity phone, worth over 200k market price and he saw the phone on Facebook and the poster was selling it for just 150k, and the phone was just 2 months old with complete accessories, according to the seller. I tried talking him out of it but he ignored my advice and went ahead to buy the phone.  Indeed the phone was very new and had rlthe receipt just as the seller posted which made the whole thing even more suspicious.

My friend was enjoying the phone not until after a month he discovered the phone was bought on instalment from EasyBuy. And we all know what that means.