: Bring a USB drive to ensure every port works; a dead port can signal deeper logic board issues or liquid damage. Test Wi-Fi and Bluetooth to ensure they connect instantly.
Unlike a new machine, a used Mac's most critical "consumables" have already been partially used: tips for buying a used macbook
: Test every single key for responsiveness. Be particularly cautious with models from 2016–2019 , which use the failure-prone "butterfly" keyboard. : Bring a USB drive to ensure every