Economy

In my area, prices at the pump vary by as much as $0.50/gallon for regular unleaded.

Really, there is no market explanation for this. These are stations within a short drive of each other, though each has it's own "location" advantage.

But there is no way two stations, with the same tax burden, regulatory requirements, and distance to distribution hubs, should be that far apart.

The only explanation is greed.

The same goes for national pricing. The refiners own the supply from well-head to delivery truck to the pump. Their costs don't vary wildly with market pricing because they don't pay spot prices. They merely use spot prices as a rough guide to set their pump prices.