If you are buying a new mobile then first asked Salim that what are the faults in the mobile then you must ask how many days he have used it and what is the battery backup of this mobile.
Check body of the mobile check if it is broken stretch or marks on screen or back panel check the display of the mobile and check overall
Open the back panel and check the screw off the mobile if it is not in its original position ( if it look like it has been opened before by screwdriver ) then I am sure that it was opened before. plz check carefully
Check IMEI number of the mobile by dialling * #06# on that mobile and verify and compare the IMEI number of mobile and the IMEI number which is on the box of that mobile if it match then it’s not a fake or theived mobile. Or just open the Google Chrome on other divice on Internet and go to the website https://imei.info and their enter the IMEI of that mobile.it will provide you all the information on that mobile that will prove that mobile is original.
Then check all the hardware functions. for checking it just install test your Android application from Google Play on that mobile and it will show you if there is any problem in a hardware or not in that mobile.
Check the battery condition if someone is selling you a mobile which is used 1 to 1.5 years old then surely its battery is weak.
You must ask for the bill and box of the mobile phone
Also check if it is capturing network or not
Check calling and charger insert sim and check calling quality sound etc and connect charger and - - charging thank you please like my review