International buyers are setting their sights on New York City real estate, with Manhattan apartments flying off the market as prime investments and pieds-à-terre for well-heeled businesspeople and frequent visitors to the city. Such is the case with the Lower East Side apartments for sale at One Manhattan Square, which have recently attracted buyers from Canada, China, France, Japan, the Netherlands, and South Korea, according to Cat Liu, director of sales at the Extell-developed tower.