Would you buy a used, slightly dusty, old bureaucracy with several hundred lawyers, a few dozen scattered prisons, a U.S. Marshals Service, its own Federal Bureau of Investigation, and a slice of prime real estate in downtown Washing- ton?

Well, the Justice Department isn't exactly for sale, but the American Civil Liberties Union is making an offer it hopes President Reagan won't be able to refuse.

The ACLU wants to buy the Justice Department.

And why not, asks ACLU Executive Director Ira Glasser, since "civil rights and civil liberties organizations are already doing much of the work that should be done by the Justice Department, and particularly by its civil rights division."

"We might as well go all the way," Glasser added.

Replied Justice spokesman Patrick Korten: "My temptation is to say, 'That must have been quite a Christmas party at the ACLU.' But I guess they don't have parties at the ACLU."

Korten added, "It must have been a slow day. It's cute, but not worthy of a serious comment."

But the ACLU was serious enough about its offer to send a letter to Attorney General Edwin Meese III. Explained ACLU spokesman Ari Korpivaara , "It's the right season for doing this."

And what price Justice? "It should be very low," Korpivaara said, "since the value of the Justice Department has fallen very low recently."

There are precedents, with the administration in the process of "privatizing," or selling off, some major programs as it pushes ahead with its agenda to get the government off the backs of the people. Conrail went on the auction bloc in the spring, and the Federal Housing Administration has been mentioned as a possible sale item.

The ACLU insists that while its offer was "tongue-in-cheek," the civil liberties group was actually making a serious point by trying to "prompt people to think about government selling off its responsibilities" and also about what the ACLU sees as a slackening of federal civil rights enforcement efforts.

Korpivaara speculated that if the ACLU takeover bid were successful, environmentalists may opt to buy the Interior Department, the education community may tender an offer for the Education Department, and the unions may make a bid for the Department of La- bor.

"Who wants to buy the Pentagon?" he asked. "Maybe Israel," he said.