The pilot reported that he was on a personal flight in a tailwheel equipped airplane that he was planning on purchasing. Having not flown an airplane with a tailwheel in approximately 10 years, the pilot flew several low approaches over the grass runway. The pilot stated that during his 5th approach he still did not feel comfortable controlling the airplane, but was slow and close to the ground so he chose to attempt the landing. On landing, the airplane began to slide to the left at which point the pilot stated that he lost control of the airplane. Still in a slide, the airplane departed the runway, struck a barbed wire fence, nosed over inverted and came to rest. The rudder sustained substantial damage. The pilot reported no mechanical malfunctions or failures that would have precluded normal flight.