Sorry to hear about the situation.
The answer to your questions does depend upon the contract you used. I am not an attorney and they may the only ones truly qualified to answer your question.
The key question I would ask is did the buyer have a contractual right to back out?
In CA, the typical contract gives the buyer a due diligence period to investigate the property. After that time period has passed, and the buyer has removed all of their due diligence contingencies in writing, then they could be liable for the default.
What recourse you have will depend on the contract? Assuming the buyer did default, you might have right to their deposit or more.
I would recommend speaking with your agent or attorney about your options.\