The serial number is a Mac's ID card. In seconds it confirms the exact model, the year it was built and the coverage left — three facts a seller can round in their own favour.
Where to find it. Apple menu → “About This Mac” shows it directly. It's also engraved in small type on the bottom case and printed on the original box. On Apple silicon Macs it appears in System Information → Hardware as well.
What it contains. Older 12-character serials encoded the factory, the year and the week of production. Since 2021 Apple has used random 10-character serials: you can no longer derive a date from them. That's why you should query Apple's database rather than decode by hand.
Check the coverage. Enter the number at checkcoverage.apple.com: the page shows the warranty expiry date — and therefore, indirectly, the purchase date — and whether the device is eligible for support. A serial that is rejected or “not valid” is a serious red flag.
Spot a mismatch. Compare three sources: the serial engraved on the bottom case, the one in “About This Mac”, and the one on the box. Three identical values and you're fine. A difference means the logic board or the case was replaced — not necessarily bad, but worth making the seller explain. MacbookCheckr shows the serial read by the system, the matching model and its year, which makes the comparison immediate.
One last reflex: a Mac reported lost or stolen will be blocked at activation. Make sure Find My is off and the seller's iCloud session is signed out before any payment.