Hello,  I am the new kid on the block looking for expert information/advice.
  My son, visiting his fiance's family, called me and said, "Dad, you need a sports car."  His fiance's uncle was selling his 72 GTO Sport Coupe.  It has a 400 engine and manual transmission, apparently only 59 made with this configuration.  Pictures show a beautifully maintained original car with immaculate leather (or vinyl) seats.  It has 80K miles and seems all original.  It does have a new clear coat paint job.  
  What I would like to know is how does one establish a fair market value.  If anyone knows what a reasonable price range might be, please comment.