"Serenity now!"
Obviously you're dealing with an angry, vindictive personality.
If you don't play, he or she will probably fade away.
The first thing you shouldn't do is open and read, and especially reply to any messages and/or emails from this seller.
Phone numbers can be blocked on your phone.
If the matter escalates to numerous additional phone calls and messages, and/or threats, or otherwise intolerable acts, contact your local police with all of the documentation you have accrued.