> Most people selling their Macs do not even know that they have to formally pass ownership on to someone else

Then the person who buys it from them is going to send it back, and they'll know next time, or however that gets resolved. If they're selling it to a reputable commercial entity, that entity will know it needs to be done, and will ask them to do it first. Every iPhone I've sold to a company in the last x years has needed this process done...