A sensuous Paul Gauguin painting of two Tahitian girls has been sold from a Swiss private collection for close to $300 million, one of the highest prices believed to have been paid for an artwork, according to European and American art world insiders with knowledge of the matter. The sale of the 1892 oil painting, “Nafea Faa Ipoipo (When Will You Marry?),” was confirmed by the seller, Rudolf Staechelin, 62, a retired Sotheby’s executive living in Basel, Switzerland, who through a family trust owns more than 20 works in a valuable collection of Impressionist and Post-Impressionist art, including the Gauguin,...