Assuming everyone could determine which gas stations sold Exxon-Mobile gasoline, everyone theoretically could buy their gasoline from other dealers. Assuming they did so, by the law of supply and demand, the increased demand among these other dealers would drive the price of gasoline up.
While the price of gasoline at Exxon-Mobile dealerships might decrease, how would this benefit us if we none of us were buying gasoline from them? When people started buying Exxon-Mobile gasoline again, the increased demand would drive their prices back up and we'd be back where we started.
