Keep this guy's contact information. Odds are in a month or so you'll be able to buy it for half his price or less. The fact is that 85 DeVilles are not collectibles, and they bear the extra curse of the HT4100' s bad reputation.
I'd also be interested in the reason why the miles are so low. Was this car a lemon from new? Keep in mind too that if the coolant hasn't been changed regularly, this car could have corrosion issues, including the aluminum block. See if there's any service records.
If you're really hot for it, make him an offer of 5g's. After he stops laughing, just say, "The offer will be good for 45 days." Bet you'll own it.
That is a real cherry of a car, it appears to be in perfect condition. I agree with noahsdad beware of the HT4100 Engine, it is a fine engine if it has been maintained with regular coolant changes with the sealer tabs. My Grandpa has an 85 Fleetwood (Just like the Deville, just different name with minor trim changes) with 26,000 miles it has had some problems because it sits and is not driven sometimes for months. They are great riding cars, and are well built. The power is pretty good because this engine was designed for the Front Wheel Drive models. Good luck, let us know if you buy it.