A bizarre case involving a former narcotics informant, the prestigious New York fine arts auction gallery Sotheby Parke Bernet and a Westchester County newspaper editor ended recently when the editor was convicted of trying to con the gallery out of $100,000.

Robert G. Manfredi, 33, of White Plains, president of the Westchester News Corp. and managing editor of eight community weekly newspapers, was convicted Jan. 12 in Manhattan Supreme Court of attempted grand larceny in the second degree. The informant, James Toohill, 34, of Scarsdale, N.Y., had pleaded guilty to the same offense 10 days earlier. Both were charged with offering to sell Parke Bernet information about an investigation the New York County District Attorney's Office was making of the gallery.

When the episode began in August 1977, Toohill was an undercover informant for the District Attorney's Office. Previously he had worked on drug detection for the U.S. Customs Service, and also claimed to have worked for the CIA and FBI. In seven or eight years of being an informant, he had earned about $14,000, Toohill testified.

Being something of an art buff as well, Toohill made certain non-specific allegations to the D.A. about financial improprieties involving Parke Bernet. Among other things, these related to the gallery's practice of putting undisclosed reserve bids on items for auction.

A reserve bid is a minimum selling price agreed upon by the seller and the auctioneer. When bidding does not reach the agreed price, the item reverts to the owner, although the auctioneer says "Sold!" About five years ago, Parke Bbernet came under fire by some art dealers who were regular customers of the gallery. They tried to get the gallery to disclose the amount of any reserve. The New York City Consumer Affairs Office held hearings and finally issued compromise regulations simply requiring an auctioneer to state in the catalogue which items were covered by reserve bids.

An official in the D.A.'s office, who asked not to be identified, confirmed that there had been a preliminary criminal investigation of Parke Bernet, but said it has been terminated. He also said that Toohill would have been paid "a couple hundred dollars" at most if he had produced evidence; as it turned out, "We never paid the creep a dime."

In October 1977, Toohill, who then had a "CETA job in arts and crafts." according to the same official, applied for a job as a photographer with one of Manfredi's papers. When he was refused the post, Toohill contacted Manfredi and suggested they could work together in another way. The informant told the editor of his work with the D.A. and promised to help him get an exclusive story.

According to the prosecution, Toohill and Manfredi then decided to tell Parke Bernet about the investigation in exchange for $100,000. The two men were to have shared the money. Starting on Oct. 28, Manfredi made nine phone calls to the gallery's president, John Marion, telling him what the D.A.'s office termed "half truths."

Marion became suspicious right away and called his lawyers, who called the D.A. His office tape-recorded calls made to Marion. On Nov. 17, Manfredi called Marion to make arrangements for payment. Manfredi instructed him to put $100,000 in a briefcase and to give it to a courier, who would meet with Toohill and Manfredi's courier in front of a midtown Manhattan restaurant. After they exchanged briefcases, Manfredi's courier discovered his contained no money but merely a copy of the New York Post. He was arrested on the spot.

Toohill changed his plea to guilty just before he was to face a jury. In his statement, he said he was working on his own behalf, aided by Manfredi, and that the D.A. had not authorized him to get a newsman to help in the investigation.

At his trial, Manfredi contended that he cooperated with Toohill because he thought Toohill was a legitimate police agent asking for his help.

Manfredi's defense lawyer said during the summation, "Bob Manfredi is ashamed of what he did. He was trying to (cooperate) to get a story. What did he need $100,000 for? He had a going newspaper business and still does." Though it did not come out in the trial, the official close to the D.S. said their investigation had revealed that Manfredi's Westchester News Corp. was losing money, its bank balances dropping.

Manfredi issued a statement last week saying he would appeal the verdict and repeating that the trial had shown that Toohill was a federal agent. The editor is to be sentenced March 1 and faces a maximum of four years in prison if the conviction is upheld.