Legal Question in Real Estate Law in California

I think you are asking what happens when a buyer takes possession of a house before the contract was signed. I can't answer this without knowing some more facts:

1. Was the contract signed eventually?

2. Did the purchase close, i.e., was a deed delivered?

3. Was any harm or loss caused by the premature taking of possession?

4. Was possession taken peaceably, or by a break-in or the like?

I have heard of couples moving in to a vacant house after the contract was signed but before close of escrow, and without the knowledge or consent of the seller. When no harm is done and no violence was used and the deal eventually closes, I don't think there is a real problem here. There has been a technical trespass, but nothing that would result in a criminal prosecution or lawsuit for damages.

On the other hand, if someone's valid interests are invaded, or if there was a breaking in, or if the deal doesn't close as planned, there could be cause for a lawsuit - perhaps for unlawful detainer or for ejectment - or maybe criminal charges could be brought.

Please provide more details if you want a better answer.