Five minutes that save you a headache:
- Find My Mac turned off, in front of you (System Settings › your name › iCloud). Otherwise the Mac stays linked to their Apple Account: it can’t be erased or reactivated without their password.
- Apple menu › About This Mac: the chip (or processor), memory and storage must match the listing.
- Screen and ports: a white, then black, full-screen image, and every port.