Nice, that links is perfect.Yeah the ideal option would be simply seeing them do Erase All Contents and Settings in front of you or doing it yourself.
What to do before you sell, give away, trade in or recycle your Mac - Apple Support (ZA)
Reset your Mac to factory settings to prepare it for the new owner.support.apple.com
If it restarts to Setup Assistant you are golden.
If they can’t complete that process they don’t have the Apple ID password tied to the machine.
Basically do that if you don't want to risk losing your money.