Before you buy 2nd hand phone you need to check few things
Phone Hitting issue ( check if the phone get too many hit or not)
Performance ( run or open multiplayer application make sure is phones get hang or not)
Cammera ( check front and back camera)
04.sim and sd card slot ( check the sim card and sd card slot setting are good or not)
Battery backup ( under the battery details, check which app use too many battery power, if its above 50% used than the battery is not so good)
Try to record some video or voice for aditional cross check
Phone body damage or not ( make sure the body of the phones is still good)
Good luck