... so the price would come down right away on the downward side - cost of replacement applied. And all of those people waiting to fill their tanks would go to the station with the reduced price, just as you have said.
OK, I'm not going to chase this around in circles, but here goes: No-one will sell BELOW the price of replacement, because if he would, he is better off simply not selling. I should have been clearer: the MINIMUM is the price of replacement.
OTOH, No-one's in a hurry to take a loss on buying gas high and selling low. THey'll wait to see that the price of gas doesn't bounce right back. No-one is going to be saying, "Woo-Hoo! I sold all my gas!" when they sold it for less than their costs, and then they have to refill for more than their sale price again.