If you are buying a second hand smartphone then the main thing is what is the condition of smartphone . if smartphone is dead dont buy it in any cost .if phone is working propely then check battery condition and phone screentouch and other mobile function by using 5-10 minute .and then decide the phone is usefull or not .and the price of second hand smartphone is effectively half in good condition .if phone is working but some function is not working then the price is less than 50% and I think buy second hand smartphone oly if the condition is good other wise go for new phone .