On July 17, 2004, about 1100 eastern daylight time, a Cessna 182, N666PG, registered to Inderluft LLC and operated by a private individual, as a Title 14 CFR Part 91 instructional flight, had a hard landing at Marco Island Executive Airport, Naples, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The commercial-rated pilot-in-command/certified flight instructor (CFI) and the commercial-rated dual student received no injuries. The airplane incurred substantial damage. The flight originated from Naples Municipal, Naples, Florida earlier that day, about 0930. The owner of the airplane stated he made arrangements with the CFI to demonstrate the airplane to the dual student. The dual student stated they departed from Naples for the instructional flight with a southeast heading towards Marco Island to conduct takeoffs and landings. During the flight, they performed steep turns and stalls for the potential buyer to become familiar the airplane. The dual student was on the second touch-and-go exercise, when during the landing the aircraft landed hard. The crew continued the flight and made one more landing with a full stop. The CFI and dual student got out of the airplane to examine the airplane for damage. They noted wrinkles in the skin of the fuselage next to the firewall section. The airplane was flown back to Naples where it was later determined the airplane incurred substantial damage by a licensed airframe and power plant mechanic. The airplane's owner had it partially disassemble and transported to a repair facility in Texas. Neither the CFI, commercial-rated dual student, nor the owner of the airplane reported the damage to the NTSB. On October 7, 2004, an FAA aviation safety inspector from the Miami Flight Standards District Office informed the NTSB of the accident. On October 13, 2004, a completed Pilot/Operator Aircraft Accident Report was received from the CFI. The CFI stated there were no mechanical failures or malfunctions to the airplane or any of its systems prior to the accident.