If you want to buy a second hand phone because your budget is less but you want to buy smartphone then there are some you should follow before buying any second hand mobile -
There are many online sites or apps like OLX, on which you can search on second hand mobile and you can see the pics and specifications on the mobile on this. There is also a mobile number given from the owner through which you can contact the owner.
Then contact the owner first whose mobile is still in warranty ( if you found any) by call them or you can also chat with them on some applications. OLX provide you this feature. And then clear all your doubts with mobile’s owner regarding the mobile you want to buy.
Then after you clarify all you questions, then meet with the owner and check mobile specs. Check that Internet connection is stable and internet runs smoothly. Then connect it with WiFi and also check its portable Hotspot feature.
Then install some games on that phone and play it and see if it heats or lacks or it hack the mobile. Send some files through Bluetooth to check Bluetooth connectivity.
Call someone to check network availability and speakers of the phone and also put call on speaker. Click some photos from camera and verify its photo resolution and clarity. Check the warranty card, mobile still in warranty.
Then buy phone after you get fully satisfied.
Thank you.