I don't know about Marin County, but I can tell you that around here there is essentially no money to be made in the retail gas business. That's why every gas station sells lottery tickets, tobacco products and snacks. That's where they actually make their money.
It's also the answer to the dumbass Bill 0'Reilly's perennial question: "Why do gas stations suddenly start charging more when the price of oil goes up? It can't possibly be affecting what the gas cost them to buy." The answer is: their margin is so small on retail gasoline sales that they will not be able to fill their tanks next week if they don't start charging next week's price right now.
And then they wait a month to lower it at all.