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.
This is true mate, they don't have the money to buy a new smart phone so they usually ended up buying a second hand or third hand phone from someone that they don't even know. I can remember fully well when I was a the road side and someone ask the next person that's closer to home that anytime he sees someone who's interested in selling his/her phone he should let him know (it's not good). And you know, if the person he told about buying of smart phone is a bad person he will go and steal from other people and sell to the person, so if eventually the owner cought the person that buys the phone, then he's in a big trouble, so it's not good to buy a second hand phone even if the phone is new and cheap. Mostly when the phone doesn't have a receipt, then one shouldn't buy it at all, I must advise a buyer to check on easy buy for something that has to do with smart phones.