+1 to Wayne…
It’s simple, just DMV’s DTF-802 form, which is basically like an affidavit of sale for private party for less than blue book value.
Important: seller of the vehicle MUST sign this form.
But basically you can put in a value lower than the blue book used for tax calculation purposes. This form can also be used where there is a partial trade for services and/or for cases where the vehicle is given as a gift.
Just an extra step in the registration / buying process but it works.