AQUINNAH, Mass. – The Martha’s Vineyard estate of former first lady Jacqueline Kennedy Onassis is being sold to a pair of nonprofits that plan on turning the property into conservation land open to the public, officials said Thursday.The Martha’s Vineyard Land Bank Commission and the Sheriff’s Meadow Foundation have agreed to pay $27 million for Red Gate Farm in Aquinnah on the Massachusetts island, Land Bank Executive Director James Lengyel and Sheriff’s Meadow Executive Director Adam Moore said in a joint statement.The below-market transaction is expected to be completed by Dec.