How to add grounds to a 120 amp panel and grounds electrical outlets
I am going to assume a few things. First the inspection you are referring to is by a home inspector and not the AHJ. I have to say I have never been very impressed with the home inspectors of this area because they are not licensed electricians, plumbers or HVAC-R mechanics. They are simply someone who is somewhat familiar with general construction and building codes, and they were able to pass a general inspection test to be qualified to inspect houses. Some are pretty good. Some not so good.

I really have a hard time believing that you have a main panel without a ground. I can believe you have circuits not grounded in a house built before the mid 1960's. If what I just said is true then your house is legal under the grandfather clause. Any changes to your electrical system is an upgrade and if the buyer wants an upgrade then he should be responsible for cost incurred or a mutual agreement between the buyer and the seller.

My advice is to hire your own electrical contractor and have him review the comments and reinspect the house. Then he can present you a letter on his own letterhead and license number stating that the system meets the code it was constructed under. He will probably also suggest to you what you really need to fix. 

I can't speak for the State of California or your local jurisdiction, but my masters license and a letter on with my letterhead and my license numbers (contractor and master) over rides any home inspectors report in this area.

PS My license never over rides the AHJ.

Hope this helps.