As a matter of law, you would not be liable for what someone else does. You would have to make sure you could prove that it wasn't you flying it, but that would be pretty easy I would imagine, unless you sell it to your neighbor or something. When you sell it, get a bill of sale and on the bill of sale (assuming you can't deactivate the NFZ) have it noted and have the person sign it they you are transferring ownership and that you are lo longer in control of, or responsible for the flight of that craft.
You can't just charge someone with a crime they didn't commit but you should safeguard yourself from any doubt. And no, they can't write in a waiver "if you ever sell it and that person...." because that would be Yuneec creating the letter of the law in this country and that's not how out legal system works. Not only do you have to be the one who did it, but you have to intend to. Almost everything that is criminal has to have intent or it is not criminal. Manslaughter, child/statutory rape and various forms of negligence are the only crimes where it intent is a non-factor.
I know I off on one of my tangents now but I suppose if you got an overzealous prosecutor if you sold your non-NFZ bird and that person killed someone and they claimed they didn't know they were in an NFZ because you waived it, then you could be held responsible by way of negligence and a death involved in that negligence would rise to manslaughter.
So I go back to my original statement. You would be fine if you cover your *** with the sale and the acknowledgement of the fact that the new owner must be aware of NFZ's on his own. Wouldn't want to get charged with manslaughter after all. Speaking of manslaughter, if you want to laugh, watch this:
I defy anyone to watch this until the end and not laugh their *** off: