In a private sale of used goods on Facebook.  You have no obligation but to be honest.  You didn't misrepresent so you have done nothing wrong.

This is well outside the jurisdiction of the police. 

IME when people get aggressive like this, it's because they have something to hide.  For example, they dropped the phone.

I'd be telling the person you have no obligations to them, if they believe they have a dispute, then the disputes tribunal is the correct channel for that.  I'd also be telling them if they continue to harass you with aggressive messages, you'll be referring the matter to the police.  That is the civil response.

The police often take threatening messages seriously.