Hello. It depends how keen the dealer is to have your A6 Avant. It's coming up for five years old, which means it's touch-and-go whether the dealer would want to retail it - assuming it's a main dealer. If he wasn't retailing it, it's likely that it would be sold to another dealer, an independent, or auctioned - which is reflected in the price you've been offered. Also, have you mileage-adjusted the Glass's Guide price? 90,000 miles is a little high (though not excessively so) for this age of car. If you haven't, you may find the Glass's price is closer to the one the dealer is offering. You'll get a much better price if you sell privately. There's a reasonably strong market for your car.