Sorry to hear of your situation which must be very frightening for you and your child. Are/were you married? If you were, have the finances been settled formally with a
Consent Order? Have you agreed the split of all other assets?
I believe that priority is always given to the housing of young children and the primary carer so would be surprised if your ex would be able to convince a judge to force a sale unless the property is much bigger than you need. Can you afford to keep up with the payments on the house yourself?
Are you saying that he''s asking you for more money than the house is worth or more than the equity?
I would be furious if my ex had sent Estate Agents around without my permission. Most estate agents would insist on a signature from both owners before marketing a property. How long has your ex been moved out? Depending how long he''s been gone you could have rights to privacy in your own home so you might need to think about changing the locks.