When officials at Peacock Buick-Opel in Fairfax County asked themselves 15 months ago, "Would you buy a used car from this man?" they said yes. What they bought was a 1975 Pontiac Grand Prix for $4,100 and a lesson in the intricades of car thievery.

The Peacock buyers did not know the car had been stolen and so they sold it to a Reston couple who in turn had the car stolen from them, possibly by the original thief. The car was later burned beyond recognition and abandoned on a road near Richmond, leaving the couple with no car, no insurance reimbursement and the loss of $800 in golf equipment in the car's trunk.

But yesterday Fairfax Circuit Court Judge Percy Thornton ordered Peacock to reimburse the couple, Robert and Margaret Ensign, now of Freeport, Fla., $5,634.40, the amount the Ensigns paid for the car, and pay interest totaling about $550. The Ensigns were not reimbursed for Mrs. Ensign's personalized, hand-made golf clubs left in the trunk.

During an hour-long trial, witnesses pieced together an intricate tale showing how car thieves dupe car dealers and ultimately the consumers who buy the stolen, used cars.

Kenneth Watkins, used car manager at Peacock, testified that a man who identified himself as Steven Paul Williams of Norfolk, Va., brought the Pontiac to the dealership in February, 1976. Watkins said he checked out the car with the FBI, which said it was not stolen, and then bought it for $4,100.

Mrs. Ensign testified she and her husband bought the car Feb. 16, 1976, and that the Peacock salesman told them he would mail them the car's title within six weeks. Mrs. Ensign testified she never received the title.

Several weeks later, a person, who said he was the car's previous owner, called Peacock and said he had left some papers in the Grand Prix and wanted the Ensigns' phone number and address so he could retrieve the papers.

Peacock asked the Ensigns' permission to give out their address. As Mrs. Ensign testified, "Of course I trust everybody, so I said 'yes'."

Several days later Mrs. Ensign left her Reston apartment one morning and discovered the car was gone, she testified.

Mrs. Ensign began crying as she described the golf clubs her "darling" husband had bought her, the golf shoes, putter, 12 new Titleist golf balls, a bll retriever and other items that were burned with the car.

R. D. Shuder, an agent withthe National Automobile Theft Bureau, explained that the car had been stolen Jan. 30, 1976, from a Virginia Beach car dealership. The thieves bought a junked 1975 Grand Prix from an auto junk dealer near Norfolk and obtained a title for it. They switched the serial numbers on the old and new Grand Prix making it appear and title belonged to the stolen car that the thief then sold to Peacock.

"(Peacock) made every effort to make sure it was a legitimate sale," Schuder said.

Peacock's attorney, Gayle Matthews, contended that there was no proof that the car the Ensigns bought was the stolen car. But Shuder said that another serial number is placed on all cars in a secret place where only a selected few people know about it.The burned-out car's secret serial number shoed it was the car stolen in Virginia Beach, he said.

The Ensigns attorney, Robert K. Richardson, attempted to show that the same thief who originally stole the car, also took the auto from the Ensigns. Thus Peacock could be liable for the Ensigns buying a stolen car and then having it stolen from them, Richardson contended.

Thornton dismissed that theory. "That's fantastic. That's fantastic. You mean (Peacock) can forsee that the vehicle can be stolen again?" he asked.

Thornton said Peacock was only liable for selling the Ensigns a stolen car, not for the second theft of the Pontiac from the Ensigns' driveway.