An 18th-century unsigned oil painting of the Grand Canal in Venice, estimated at a modest $6,000 to $8,000, sold for $687,125 Sunday afternoon at Sloans & Kenyon auction house in Chevy Chase. It is believed to be the most expensive painting ever sold at a Washington area auction. Thirteen phone bidders competed against live bidders in the gallery for this work from the "school of" the 18th-century artist Giovanni Antonio Canaletto.The consignor's grandmother bought the work in 1881 while on a Grand Tour...