Before buying a used mobile first set your budget and then look fot its reviews on youtube and then make the final mind for one set and do follow the below guide line to be safe
Before buying a used phone please make sure to check its IMEI number by just simply dailing * #06# in the dailer menu and after checking it insert your sim card and memory card just to check that the slots are working .
Here I give you the bonus trick that you can check all the features by just dailing a single code that is * #0* # and after dailing this code a screen will apear with bundle of teating features .
Good luck and lasting check headphones and charging jack.
good luck