With Billy Carter selling off historic family correspondence to an autograph collector, there is another fascinating bit of Carter administration memorabilia moldering in the Justice Department files that might someday end up back in private hands and up for bids.

Government prosecutors have in their possession a desk blotter seized by subpoena from the Washington office of Edwin P. Wilson, the former CIA operative who fled the country in 1977 to avoid prosecution on charges of running a secret training school for Libyan terrorists.

Wilson, who has acted as an agent for Libya's Col. Muammar Qaddafi for the past five years, is believed by federal authorities to be living in Tripoli under his benefactor's protection.

Left behind in his haste, according to sources, was the blotter, filled with doodling. Apparently in Wilson's handwriting, say the sources, are the doodled words "Billy Carter" and "Charter Oil."

The doodling appears to have been done long before the dates in Novemeber and December of 1979 when the FBI first learned through intelligence sources that Billy Carter was negotiating both a loan from the Libyans and an oil deal with Charter.

Wilson's name never surfaced in the Billy Carter investigation, which ended with the former president's brother registering himself as an agent for Libya.

Now, suddenly, Wilson's name is "hot" with the media again, since a former associate, Kevin P. Mulcahy, himself a former CIA agent and the son of a respedted agency official, decided to go public with the story of how he tried unsuccessfully for years to get Wilson brought to justice.

Mulcahy, who has been telling part of his story to The New York Times, is negotiating the sale of a book to Random House, and all the networks have been after him. The only show he's accepted so far is an upcoming segment on the "Today" show.

While Secretary of State Alexander Haig is making headline with his China trip, National Security Adviser Richard Allen, who is known for keeping a low profile in the administration, is also off on a trip.

Allen said he was leaving yesterday for Plains, Ga., to brief Jimmy Carter on national security matters. "It's my responsibility to brief former presidents," he explained, adding that in the future he will probably send an aide or "just send the material."

After Plains, Allen said he was planning to scoot up to his summer house on Long Beach Island, off the coast of New Jersey, for the weekend.

Attorney General William French Smith has only praise these days for his staff at the Justice Department.

Talking with Reagan's political adviser, Lyn Nofziger, at a party Tuesday night, Smith said, "We've got some great guys over there [at Justice]."

But Nofziger, ever the joker, countered: "As a matter of fact I was planning to call you over to the White House tomorrow to tell you about the bad guys you have at Justice."

"Well the bad guys we have came from Pen James' ship," Smith said, referring to White House personnel director Pendleton James.

"It doesn't matter where they came from, they ought to go," Nofziger joked.

Later on, during another conversation at the book publication party for Ed Klein, editor of The New York Times Magazine, Smith got another chance to tout his new staff.

"I've got a bunch of top-flight people [over at Justice]. The type of people I'd bring back with me to my law firm when I leave. I'm really just recruiting here," he joked.

Times columnist William Safire hosted the party for Klein, whose new spy novel, "The Parachutist" is just on the market. Klein said the characters are a combination of fact and fiction, and that CIA Director William Casey might recognize himself among the characters.

Former CIA director Richard Helms and his wife, Cynthia, joked that they were there "because our air conditioning isn't working and we had to leave the house."

Cynthia Helms, who said she is still surprised at the interest in her book, "An Ambassador's Wife in Iran," had just gotten word that a group of Persian folk stories she translated into English will be published by a British firm. The stories are traditional folk tales for children.