This is just a warning. My G/son {a student} purchased a used Apple PC which failed within the 6mnths warranty. The well known high st used goods shop [yellow paintwork} spotted 4 of the minute screws holding the case together were missing. Mine are very recessed,hard to see. They refuse to honour the warranty. This is ongoing,as £500 is a lot to loose.
So be sure to take a magnifying glass with you,if you should be tempted in.