I doubt it is titanium. It may have a Reynold TI decal on it which may lead him to think it is. Usually people see Reynolds and think aluminum.
Ask him for the serial number on the underside of the bottom bracket and then go to
the vintage Trek site to find out what it is. They have the serials there. A quick check of the brochures there show a 500 model in 84 with Reynolds frame.
$100...fair price...offer less.