In California, if they do not have a valid trust, then most likely you will need to file a petition for probate, even if you have a valid will.
Estate Sales are more nuanced than standard real estate sales. You should work with someone who specializes in Estate Sales.
Take a look at this video of my interview with Probate and Estate Attorney Charlene L. Usher discussing why working with a Realtor Who Understands Probate is Essential

Are you looking to sell the property of a deceased friend or family member?