TerraCycle Inc. aims to make money by reusing the hard-to-recycle trash the U.S. produces each year — but it first needs to find out if Walmart Stores Inc. and other retailers think there's enough demand for its products.    The company, which sells backpacks, kites and insulated coolers made from reused candy wrappers, drink pouches and potato-chip bags that normally would have gone to landfills, has so far been a money-losing proposition. But Walmart, the world's largest retailer, agreed to sell dozens of TerraCycle products in about 3,400 stores in a promotion tied to Earth Day during the month of April.   If TerraCycle sold enough to land an extended deal with Walmart or another big retailer, the Trenton, New Jersey, company could turn its first profit this year. "The pressure is as high as I can think of," says founder Tom Szaky.