May I asked what is the price of that A6 Quattro you plan to buy ??? What entine size/class -- is that a 2.8L V6 with 30 valves or 2.7 Twin Turbo engine or even a 4.2 L V8 engine ???
Usually, go to
www.kbb.com check their resale value will give you a general guilde line of private party or private party sale.
At 80,000 miles for the A6 Q, I think that is a bite high on mileage use. Just my 2 cents.