Why do you think that the dealer has an obligation to provide you with the service records of a car that you do not own, absent the owner's permission? I'd be extremely angry if either my mechanic or my dealer provided information about any of my car's service history to a stranger w/o my prior, written permission. If the owner won't give you permission to check the car's service history there is nothing that you can do about it. Plain and simple, end of discussion. The dealer has no obligation to you, only to the car's owner. Maybe the owner is trying to hide something, in which case your best bet is to look for another car. But under no circumstances should a dealer give out that information w/o the owner's permission.
You may, however, run a CarFax report. It won't tell you anything about the car's service record, but it will give you a car's salvage history (if any) and it will also tell you whether the car ever failed an annual smog inspection (which could give you some hint about the condition of the car's fuel-ignition system).
Also, you may ask the owner to let you take the car to a diagnostic mechanic of your choosing who can evaluate the car and see if there are any problems. I trust and rely on my MB mechanic and before buying a car I take it to him for his A-OK. Ultimately, an inspection by a mechanic that you know and trust is probably a far better indicator of a car's future reliability than service records. Again, if the owner won't let you do that, walk, run, away from that car.
The above is my own $0.02 worth.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!