A used Mac can be a great deal — or a trap. Before paying, take ten minutes to inspect the machine, powered on, in front of you. Here are the points that matter most.
Battery first. It's the part that wears out and is expensive to replace. Open System Information → Power and check the cycle count: beyond 800, the battery is near end of life. MacbookCheckr also shows the remaining maximum capacity: below 80%, negotiate the price or walk away.
The serial number. Compare the serial engraved on the bottom case with the one shown in “About This Mac”. If they differ, the logic board or the case was replaced — ask why. Also check coverage at checkcoverage.apple.com.
Display, keyboard, ports. Show a plain white then black image to spot dead pixels and bright spots. Type a few lines to test every key. Plug an accessory into each USB-C / Thunderbolt port: a dead port is hard to fix.
The full check. To forget nothing, run MacbookCheckr on the machine: the app tests battery, displays, Wi-Fi, Bluetooth, storage, camera, audio and ports in one pass, then gives a clear report. It's free and works offline — perfect at the point of sale.
Finally, require the seller to sign out of iCloud and turn off Find My in front of you: a Mac still linked to an Apple ID is unusable.