What is California law on selling a used car to a private party? Is there 3 day recession policy or once it is sold no returns.

My understanding is that any private transaction anywhere in the US or Canada is as-is no returns unless a specific contract is written up that says otherwise.

It may be the same in most other countries too.

In any state the rule is you bought it, its yours unless you could show deceiption on the sellers part. Even then you'd have to sue to get your money. You can't just change your mind like you could with a door-to-door unsolicited sale.