The pilot was landing his tailwheel-equipped airplane at an off-airport beach landing site. The pilot said that, before landing, he checked the wind conditions with pilots on the ground at the remote beach site, where he had landed many times before. He said that, after the airplane touched down, he encountered windshear in which the wind direction changed to a tailwind that got up under the airplane's tail. The pilot said that the airplane suddenly veered left, did not respond to his control inputs, and was not in a position that would enable a go-around attempt. He said that the airplane flipped over, sustaining damage to the landing gear, wing, vertical stabilizer, fuselage, and propeller. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.