Buying second hand phone no dey advisable because you can't tell where the phone is coming from. Many people don enter problem by buying second hand phone that was a stolen one. Apart from second hand phone been a stollen one sometimes e fit be phone wey dey very faulty and buying it to use will cause lot of stress repairing the phone always. The issues that comes with a second hand phone is very complicated.
The reason some people buy second hand phone is because they do not have the money to buy a new phone. Second hand phones are cheap. But it would be better to buy a used phone from a phone seller instead. OP is right because he said buying it from an unknown person. If I know a person very well and I do not have enough money to buy a new phone, I can buy phone from the person especially if he wants to get a new one and want to sell his phone. But if the person that want to sell phone is not known, that is not a good idea. But you are also right that second hand phone can not be good as new but not everyone can afford new phone of certain specifications that they want.
Yes all you have said has nothing to argue about it for that speaks volumes in my heart, there no body that is given a brand new phone or have all it takes to afford new phone that will prefer second hand considering all manner of risk involved in anything second hand weather from known source or not including durability issue, everything is just matter if financial incompetent of the the buyer, but in my opinion who ever that have the plan of buying a phone show plan ahead before time and save gradually as that will usher the buy good phone and save him some risk of falling onto wrong hand of stolen phone or even buy phone that is not durable.