Welcome to NICO Club!
If I was into trucks, I'd be right over! I'm in Monterey and '77 is a special year for me.
My condolences to you and your family for your loss.
Advertising with NICO classifieds, Craigslist, Autotrader, local classifieds and signs in the windows seems good to me. For value, see what you can buy another one for, adjust for condition and location accordingly. A little clean up, fluid refresh and some good photos will make a good ad. Finding any applicable records will add to the value. Stock is great and being a CA car, rust should not be an issue. Back registration is a small issue to resolve. They're a bit old and a bit rare, but not really that special to my knowledge. Though they are one of the earlier mini-trucks to be 'pimped out' back in the day. KBB might be used to generate ratios (scale factors) to apply to some numbers, but they'll be too low to use the actual values.
What's the data on it? Mileage, transmission, runs now or not (fire it up! the engine needs it), etc.
Here's some data to assist:
https://bringatrailer.com/listing/1977- ... 20-pickup/
https://jalopnik.com/for-5-800-get-into ... 1590505735