Fairfax County police said yesterday they have arrested 74 people, are seeking 20 more and have recovered about half-a-million-dollars worth of stolen goods in a "sting" operation they said was among their most successful law enforcement efforts ever.

"This project has netted more criminal suspects than any other single . . . program in Fairfax County," said Police Chief Carroll D. Buracker. He said the suburban fake fencing operation, modeled on "stings" pioneered by D. C. police in cooperation with the FBI, not only aided in the solution of burglaries in Fairfax, but helped police in other Virginia counties, Maryland, West Virginia and North Carolina.

The 18-month undercover operation was called Operation Kaleidoscope. Police canvassed Virginia, North Carolina and West Virginia yesterday searching for the 20 additional suspects.

Plainclothes officers operating out of a rented warehouse in Springfield and a store in the Barcroft Plaza shopping center near Baileys Crossroads bought more than 300 stolen items, including 15 automobiles, several dozen office typewriters and numerous pieces of jewelry and silverware.

All the transactions at both locations were recorded by hidden videotape cameras.

The videotape evidence has already enabled the county to win convictions in all 42 cases brought to trial so far, said County Commonwealth's Attorney Robert F. Horan Jr.

Horan said secrecy about the operation was maintained despite the trials because the taped evidence was so persuasive that there were guilty pleas in about three-fourths of the cases and the evidence didn't have to be displayed in open court.

The two phony fencing locations were called the "Barcroft Bargain Center" store and the "K and B Associates" warehouse, the latter bearing the initials of both Chief Buracker and former Chief Richard King, who is now deputy county executive for public safety. The operation cost $300,000 in county funds and $350,000 granted by the U. S. Law Enforcement Assistance Administration and the Justice Department.

Police found that it paid to advertise. They placed ads in local newspapers announcing: "We Pay Cash" for items such as cameras, watches, jewelry and office machines. The response was half a dozen customers a day who wanted to sell stolen goods, along with others with more legitimate aims. The undercover officers lived up to the ads by paying about 10 percent of the value of the merchandise offered to all comers.

But they seldom paid in cash. They used checks drawn on a Northern Virginia bank because they feared that some of their "customers" might rob them if cash were on hand.

Although D.C. police and federal law enforcement officials have staged several widely publicized "sting" operations since the first one in 1976, few of the suspects filmed by Fairfax police appeared suspicious of the county's undercover fencing operation.

In one videotape sequence shown by police at a press conference yesterday, a man bragged that he could obtain cigarettes, camera film and beer for the undercover agent.

"I can get you a lot of things," said the man, who police said was employed at a local drugstore and has been charged with grand larceny. "In any retailing business you've got to trust somebody. Too bad they had to trust me," said the man, referring to his employer.

Chief Buracker said suspects told police where they got the stolen property and most of the stolen items, including eight of the 15 cars, have been returned to their owners. He said the rest of the loot is being held until county prosecutors finish trying the remaining cases.