The first ship carrying natural gas from the Gulf Coast is expected to depart soon, marking the emergence of the U.S. as a major exporter and the globalization of the once highly regionalized gas trade. But the debut of a shale-fueled gas-export industry is raising questions about whether too many sellers are chasing too few buyers, deepening another boom-and-bust cycle. The price of liquefied natural gas has dropped 50% over the…