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 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.