Airline discount coupons - the certificates that can cut air fares in half - are being bought, sold, bartered, scalped, and now counterfeited in a national phenomenon that began as a simple giveaway gimmick.

With demand far outstripping supply and the going price for the coupons hitting $55, the craze this week took a new turn with the arrest of an Illinois man who police said had 11,240 bogus coupons printed at a hometown print shop.

The arrest has touched off a debate among airline officials, law enforcement authorities and lawyers over whether the tiny certificates, which can be as valuable as money, are illegal to reprint.

Police accused Vincent Celeste, a self-employed carpenter in the town of Sandwich, Ill., of violating a state law by ordering the coupons printed. He was charged with making documents capable of defrauding and possession of a document capable of defrauding, crimes that carry five year prison sentences plus fines of $10,000 under Illinois law.

Celeste's lawyer yesterday was challenging whether the charges were properly drawn."We're not sure a crime has been committed," said lawyer Ronald Cook in a telephone interview.

But DeKalb County police said they were confident they had a case against Celeste. "We believe there was going to be a fraud," said detective Michael Alsup.

Airlines officials conceded they were troubled by the arrest, the first known incident of coupon counterfeiting they said had surfaced since United and American airlines began offering the coupons in a summer promotion, American's security manager, Herbert F. Brick, said the bogus Illinois coupons were "of very good quality" and added that a "substantial number of them would have slipped through" undetected by ticket agents.

The airlines had hoped to forestall counterfeiting efforts by printing serial numbers on the coupons, which are about half the size of a dollar bill and were distributed without cost to airline passengers during a promotional period.

The numbers may have helped tip police about the Illinois coupons. Police said they became involved in the case when a printer in Sandwich about 60 miles west of Chicago, was unable to print the seven-digit serial numbers. Celeste then took the order to another printer in Chicago who did part of the work, according to police.

He was arrested in Sandwich, police said, shortly after he had picked up his finished order of 10,000 coupons, plus an overrun of 1,240 coupons, from the first printer. Caleste paid $25 for them, police said.

Coupons selling has become a big sideline among some Washington travel agents and yesterday some of them said word of the bogus certificates wasn't surprising. Martha Snyder, of Washington Omega Travel agency, which buys and sells coupons said, "We've heard daily to watch out for bogus coupons."

"We've been told to look at the serial number to make sure it's embossed, not printed, "she said. "I'm sure we'd be able to tell the difference."

Silver Spring coin dealer Bob Williams who also deals in the coupons, said he purchases only small quantities of the certificates to avoid the risk of being taken by counterfeit coupons. "I want to know exactly where they came from," he said.

Williams, who said he was recently assured by United and American officials that no bogus coupons were in circulation, described the coupon market in Washington as "moderate." He is currently purchasing the certificates for $45 each and selling them for $55, he said.

Since the coupons can cut as much as $225 off the costs of a round trip flight to the West Coast they are considered a bargain by many even at the current price. The coupons must be used on American or United flights before Dec. 15 when the offer ends.

United said it distributed 2.1 million of the coupons and American said it gave out 1.8 million of them.

United spokesman Doug Guinard said the airline is depending on agents to spot any bad coupons and expressed little concern about passengers attempting to fly with counterfeit ones. "Let's put it this way," he said, "United is honoring all coupons."