A major energy company will soon sell U.S. oil abroad without explicit permission from the government, another sign that the decades-old federal ban on crude exports is crumbling. BHP Billiton ’s deal to sell about $50 million of ultralight oil from Texas to foreign buyers without formal government approval is likely to be only the first of many such moves as energy companies seek new markets and higher prices for the surge of crude now pumped in the U.S.