On April 25, 1998, at 1230 eastern daylight time, a Piper PA-28-140, N3757V, was substantially damaged when it collided with the ground during a high speed taxi at a private grass strip near Downsville, New York. The certificated private pilot was seriously injured while the passenger was not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The local, personal flight was conducted under 14 CFR Part 91. The pilot said that he was demonstrating the airplane to a prospective buyer. He said he elected to demonstrate high speed taxi runs, operation of rudder control and brake control and not takeoff because the winds were gusty. He said that the airplane never got airborne. A witness, a commercial pilot, who accompanied the prospective buyer to the airport, observed the demonstration flight. He reported that the winds had decreased, and the pilot decided to take the buyer up in the air to demonstrate the airplane. He said that the pilot was taking off on runway 6, and that the winds had shifted 180 degrees, and the pilot was taking off with a tailwind. He said the airplane got airborne about 30 feet above the ground, with a high angle of attack, when the wings rocked back and forth, and the airplane touched down hard. The airplane veered off the right side of the runway, and crossed an access road down an embankment. A Federal Aviation Administration Inspector said that the pilot did not have a current biennial, and the airplane maintenance records showed that the last annual inspection was completed 4 years ago. The pilot had reported over 4,340 hours of total flight experience, when he applied for a third class medical, that was issued on March 22, 1995. He said he had over 550 hours in make and model, and had not flown within the last 90 days. The pilot reported that there was no mechanical malfunction and that the accident could have been prevented if he had taxied slower.