Depending upon how you drive, if you go by Marshall Road from Tomales you can stop at Hog Island for oysters. You can eat them there on the deck or pack them to go. If you are in Berkeley on a Saturday morning you can buy same oysters at the Berkeley Farmers' Market (same deal, take away or have them open for you on the spot.) And you can also get them at the SF Ferry Building.
I have never eaten at Osteria Stellino in Pt Reyes Station. There is of course the well worn and well known Station Cafe, just opposite from Stellino. Atmosphere hasn't changed in a million years. The food has never been great imo but it is an institution, and if the weather is warm and sunny, sitting in the back patio is pretty sweet. Excellent root beer float.
Saltwater Oyster depot is on my list of to do's--it sounds great. That's in Inverness, I believe, just a short hop from Pt Reyes Station. A great day outing: go through Inverness and stop at Saltwater Oyster (or vice versa), continue on to Abbott's Lagoon parking lot. The walk is lovely, about a mile in to the lagoon and a little more to the ocean. Great for birding, usually lots of pelicans and assorted shore birds, also great for taking a picnic. Last time I was there I saw a coyote walking on the opposite shore of the lagoon.