What makes you think oil companies will sell oil and oil products to you for less than they can sell them to another country?
Oil production is running close to limits. Oil demand is high, and rising. There are threats to the medium term supplies (eg Iraq, and especially any action against Iran)
All those go to make oil expensive. People are prepared to pay high prices, and the owners of the oil will naturally sell it for what they can get. If you want to cut prices, cut demand, increase supply, or remove threats to future supply (or all 3).
The key would be organization and communication; everyone in an area is told/agrees not to buy from Brand X until their price is 8 cents cheaper than their competitors.
Good luck cutting 8 cents out of the refining/distribution/marketting side of the equation. The problem is the costs in those areas are very low, and profits are low, as well.
The profit is in supplying crude at the moment. Crude oil is very expensive, and if you own a well, you can sell the output for far more than you could a few years ago. Hence, big profits.
But if you want to cut those profits, you have to work out a way to make people sell the US oil at less than the market rate. You have to make them say, ok, we'll sell you oil at $50 a barrel, and say to the Chinese, Germans, Japanese etc no, we won't sell it to you for $70 a barrel.
Why should they?
If the price of housing goes up in your area, do you sell your house for the market rate, or sell it for what you paid for it?