What is your offering price?
I would figure a clutch replacement is due pretty soon. You might be able to get 10,000-20,000 miles out of the clutch yet, but age can take it's toll too on the pressure plate, etc.
How bad is the paint? Would a good detailer be able to get the paint back to a decent state? Even if the paint is showing it's age, the car is worth more in it's original paint job from Germany.
You have to assess the basic mechanical issues with these cars to see where you're at. Has the alternator, starter, brakes, shocks and other mechanicals been refreshed recently? The 100k mark means that some of this stuff may be due for replacement either from failure or just preventative maintenance.
If the car carries it's original paint in fair to good condition, has a very good set of records, does not have any major oil leaks or major mechanical issues (they all have SOME issues though), has an intact and presentable interior, $19k is a good asking price. Maybe $16,000 to $18,000 is a good selling price point depending on what it needs.
If it needs a full repaint, needs a clutch and other major mechanical issues, $15,000 to $17,000 might be all it's worth... 1987 to 1989 911's are desireable cars and continue to carry a premium over the early 3.2 915 equipped cars.
Good luck,
Jay
90 964, 84 3.2