The founder of the Liquor Barn chain led a $46-million cash buyout of Cost Plus Inc. Annual sales at the Oakland-based Cost Plus, which sells imported household furnishings and other items, are about $80 million. The company operates 29 stores, including 26 in California. It was founded in 1958 by William Amthor, whose family still holds the majority stake in the privately held chain. Buyers include Bechtel Investments Inc., the independent investment arm of San Francisco-based Bechtel Group; Peter Newton and his wife, Su Hua Newton, and Steven C. Boone, who developed and managed Safeway Stores' Liquor Barn division until last year.