Hello XXXXXXXXX,
You said you signed a proposal. Did you complete a contract? An offer and acceptance, or an agreed upon proposal is not the same thing. What exactly did you sign?
That guy is trying to intimidate you. What you signed with an agreement to make a contract. Neither of you fulfilled the agreement (no contract) so tell him to go away and leave you alone. No contract means no valid sale or anything else. I would do this, however--Write all concerned a certified letter and tell them the agreement fell through, you are not selling and leave you alone.
Good luck.
Lawyer (JD)
Juris Doctorate Degree