you can use youtube to check the comparison between SLCD and AMOLED
both are good, AMOLED has its own advantages.
while purchasing a used phone, check if the charger is charging the phone, make a call and check mic and speaker, check software version (if needed), power on off, check screen if touch function is working for the complete area, most important, if you are buying from a shop ask him to provide 1-2 days warranty so that you can check the battery timing and any other fault..
this is what i do most of the time
