The best salesman at the American Booksellers Association Convention may prove to be former president Jimmy Carter, who addressed the booksellers at breakfast today on his forthcoming "Keeping Faith: Memoirs of a President" to be published in hard cover by Bantam in November.

Carter stressed that the book would be intensely personal, that it has been painful to write and that it would focus on the emotional strains as well as the rewards of being president. "Keeping Faith," he assured his listeners, also would be an account of what it is like for an ordinary man, raised "on a farm," to become the most powerful elected official in the world. "I've tried to bring the average reader into the world of the presidency," he said.

Carter reached out for the sympathies of the booksellers--and ultimately their customers--with a somber but often moving account of how difficult it was to be president and how painful he had found it to relive his trials in order to write the book. He seldom smiled and at times actually seemed distressed by the memory of events, particularly those surrounding the hostage crisis.

Carter and his publisher are clearly hoping that the former president will have more success in the bookstores than he did at the polls in 1980, and that Carter's sincerity and self-revelation, which became a liability for him as president, will be just what readers want in an author. The book's first press run will be 100,000.

The booksellers responded with enthusiasm. "He's going to express his feelings whether you agree with him politically or not. He's a warm human being--like your next door neighbor," said Chester Mann, a bookstore owner from Plantation, Fla., one of the 17,000 sellers and book people registered for the ABA gathering at the Anaheim Convention Center, on the edge of Disneyland. "As a bookseller, I'm excited. People will want to read it."

Carter, giving the booksellers a taste of what will be between the covers of his $22.50 memoir, told them, "I take the reader with me to Camp David, with Sadat and Begin, where I insisted on going when all my aides told me not to. I try to convey the difficulty of meshing two starkly different men into a common desire for peace."

He then recalled how, during the 13-day negotiating session, 10 days of which Begin and Sadat refused to speak to each other, he had awakened once at 3 a.m. with the intense feeling that Sadat was in danger from someone in the Egyptian delegation. He said he called his assistant for national security affairs, Zbigniew Brzezinski, and the head of the Secret Service detail to his cabin and gave orders that security should be increased around Sadat.

"The next morning I was very relieved to see Sadat come out of his cabin to take his four-kilometer walk. And a few days later I thought perhaps I had been foolish. Then after Sadat's assassination, I thought maybe I hadn't," said Carter.

His problems surrounding the hostage crisis dominated Carter's remarks to the booksellers. He said the most difficult point in his term came when he heard the news that the rescue mission had been aborted and that American servicemen had been killed in the transport and helicopter collision in the desert. When he and Col. Charles Beckwith, the mission leader, first met after the mission, they both "wept like babies."

The most difficult portion of his book to write, said Carter, was the account of the 1980 election. When Khomeini reversed the Iranian Parliament's decision to release the hostages on the eve of the election, "We knew we had lost," he said. "I don't like to get beat. No one likes to be rejected by the people that we love," he told the booksellers.

"Keeping Faith" was drawn from a 5,000-page diary Carter kept as president. In addition he had access to 25 million documents that constitute his presidential papers. Carter said he had read neither the Ford nor the Nixon memoirs before beginning his own. However, he met in March 1981 with a group of historians at Princeton who advised him on what shape his memoirs should take and what they should include.

"I really put my heart into this book," said the former president. "It's been a lot of work, but I've enjoyed it. I didn't want to waste a year of my life on a book no one would read. And I'll work with Bantam to promote the book as much as my schedule will allow."