No, if you properly reset the machine (which logs you out of iCloud, turns off find my Mac, etc), there’s no problem in giving the used machine to someone else.
It only fails if you don’t log the machine out. This is to deter theft, since a reset requires the user’s password. If it’s not been reset it’s a brick.
Yes, but if you’re the original owner that shouldn’t be an issue.
The other thing to note is that if the computer hasn’t been signed into iCloud or had Find My Mac turned on, none of this necessary. So someone who’s paranoid about phoning home can probably resell after a simple reformat.
It only fails if you don’t log the machine out. This is to deter theft, since a reset requires the user’s password. If it’s not been reset it’s a brick.