Won't you spend as much on gas driving to wherever you decide to bulk shop (Bay area isn't tiny) compared to your neighbourhood supermarket, as you'd save by bulk shopping?

It really depends on what you are buying and the nature of the 'market'. It's not only bulk buying either; if you buy in a different way, you can save money depending on the market.

An example I am involved in is with wine. It is cheapest to buy wine by the case, en primeur (basically buying in advance). Yet many wine lovers don't want a whole case. So consortia are set up on wine forums to buy several cases and split them across tens of people. They are often setup when one member of the forum spies a 'parcel' of wine going for a good price at a reputable retailer. This can typically save around 40-50% and given these wines are generally £15/bottle, and in some cases a LOT more, the savings are pretty good. Normally delivery is arranged to go to one person in a given area, who then distributes to others or waits for the others to pick-up.

Buying clubs are surprisingly effective depending on what you're buying. I have friends in clubs that cut their non-perishable food costs in half.

Try Berkeley.

Now that's something I could buy into