The U.S. Court of Appeals here yesterday reversed the conviction of one of the regular customers of the Sting undercover fencing operations, saying the trial judge should have let the jury consider his defense that he was lured into selling a gun to undercover agents.

The 2-to-1 ruling is the first appellate reversal of a criminal case growing out of the three undercover fencing operations that operated here over the last two years with staffs of law-enforcement officers.

The case involves James B. Borum, who visited the Sting operation 27 times during a three-month period, sometimes dropping in twice a day to sell allegedly stolen goods.

Borum pleaded guilty to receiving stolen property and selling stolen government property, but went to trial on charges that he sold a gun to the agents. He contended that he sold them the gun only after they refused to buy other property from him, after they discussed guns with him about 50 times and only because he thought they were dangerous underworld figures who might be more receptive to him if he sold them guns.

The trial judge ruled that the jury could not be instructed on the entrapment defense, which can be raised when undercover agents "induce" someone to commit a crime who is not "previously disposed" to commit the crime.

The government contended that the agents permissibly "solicited" guns from Borum instead of inducing him to sell them a gun.

U.S. Circuit Judge Harold H. Leventhal, writing for himself and U.S. Circuit Judge Edward A. Tamm, pointed out that Borum told the officers several times that he was unwilling to deal with guns and there was no evidence that he had dealt with guns in the past.

In fact, Borum testified, he stayed away from guns because he knew a conviction on a gun charge carried more severe penalties than on other types of property.

U.S. Circuit Judge George MacKinnon disagreed with the majority, calling Borum a "confirmed burglar" who was predisposed to sell anything for "a safe and good price."

He said the role of the agents was "significantly passive" in dealing with Borum, whose reluctance to deal in guns was "not based on moral principles and was easily overcome when his own stupidity induced him to perform such acts."