Buy second hand phone only from trusted reseller or any known person , cause you dont know what is the cause of user to sell it maye be it have some defects or some bugs
So buy only from know person
Some people also consider site for that
Like amazon, flipkart, ebay.etc
But I dont think that is a good choice
. You dont. Know that person
. You dont know the cause of resell
You can consider these facts