A former CIA employee accused of trying to sell classified documents to Soviet officials last December actually thought he was on a secret assignment for the CIA, his attorney contended in opening remarks to a jury today.

Edwin Gibbons Moore II, 56, was recrited during a trip to his father's home in Elm City, N.C., last year for the unidentified espionage project by "an individual whom we will not be able to produce" in court, his attorney, Courtland K. Townsend Jr., told the federal jury hearing the case.

The lawyer said the unidentified individual later provided Moore with the hundreds of documents containing thousands of pages, many of them marked secret or confidential, which were later found by Federal agents at his home, 4800 Fort Summer Dr., Bethesda.

Moore, whom Townsend said has a history of "mental instability," agreed to the project because he thought he would be "assisting the (Central Intelligence Agency, aiding his country, and would be paid money."

Moore has pleaded innocent by reason of insanity to the five counts against him. If convicted of the most serious, trying to injure the national interests of the United States by selling classified documents, he could be sentenced to lif in prison. Moore also faces 10 years in prison if convicted on any one of the four additional counts of unauthorized possession of classified material.

Assistant U.S. Attorney Thomas L. Crowe used much of his opening remarks to read from the typed note Moore allegedly dropped inside the gates of the Soviet reisdence located at 3875 Tinlaw Rd. NW last Dec. 21.

"'I am offering your organization penetration inside (CIA) Headquarters . . . for a certain fee'," of $200,000, Crowe read to the jury.

The poorlytyped note, which was almost illegible in places, also promised to provide to the Soviets the name and location of 5,000 CIA employees around the world, as well as the locations of safe houses, defector processing centers, and Voice of America and other communications transmitters in this country, Crowe said.

The writer of the note also promised to give the Soviets the names of other "potential sources" within the agency who could in effect be counterspies for the Soviets because of their own personal disenchantment, Crowe said.

Along with the note, Moore also allegedly dropped into the Soviet residence photocopied portions of a January 1973, CIA telephone book stamped secret, and a complete photocopy of an April, 1973, secret CIA phone book, according to the indictment.Crowe told the jury of nine women and three men that, even though these documents were three years old at the time, they were alldgedly offered for sale and they still contained valuable information.

To buttress this position, Crowe said that later in the week he would call former CIA director George Bush as an expert witness on the documents.

Crowe also said he would call John Barron, the author of a book on Soviet secret police, and Robert Gambino, the CIA's director of security, as additional witnesses. A fourth unidentified but "very knowledgeable" witness might also be called to testify against Moore, he said. He would not elaborate.

In his opening statement to the jury, defense attorney Townsend argued that Moore was used to dealing with the CIA in unusual ways. As an example, he cited an incident in 1967 in which he claimed Moore was paid $38,000 in back wages by the CIA.

Moore resigned from the CIA in 1961 when he was charged with arson in North Carolina, Townsend said. After three trials on the same charges, he was reinstated in the agency. It was at that point that he was given back wages, Townsend claimed. The paycheck was drawn on a private checking account in a New York City bank rather than on a CIA pay account, Townsend said.

The trial is expected to last at least one month and is being presided over by U.S. District Court Judge Frank A. Kaufman.