On a chilly Wednesday morning, about 300 people gathered at the Washington Navy Yard hoping for a bargain in an auction of 75 used government cars and other vehicles.

Bidding began promptly at 11, and minutes later George Walls, Smithsonian museum shops operations manager, became the delighted buyer of a 1976 American Motors Hornet, a dark green 4-door sedan:

* Mileage: 37,345.

* Appearance: Excellent.

* Tires: Good.

* Past use: Probably in the federal motorpool fleet.

Price: $1,400--and, Walls figures, at least several hundred dollars less than he might expect to pay at a used-car lot.

At numerous locations throughout the country, federal agencies regularly put up for sale used or unneeded vehicles--and almost anything else you might imagine. The Defense Department and the General Services Administration account for the bulk of the offerings. Defense alone received $122 million from 1,600 sales last year.

"We sell everything that comes into the government--from buttons to battleships and aircraft carriers to airconditioners," says spokesman Bill Collins of the Defense Property Disposal Service in Battle Creek, Mich.

Adds Earl Sexton, of the Defense Department's eastern regional sales headquarters in Columbus, Ohio:

"We get everyone from an individual who wants a hammer for Aunt Suzy to someone looking for a machine tool."

Much of the surplus is simply junk, bought by scrap and salvage dealers. And the watchword is buyer beware: The government is most reluctant to take anything back. Generally, it is sold by auction, spot bids or sealed bids.

But a diligent (or lucky) searcher is likely to encounter good buys on many ordinary household goods, from refrigerators and furniture to typewriters, tools and maybe a sailboat. The U.S. Customs and Postal services also hold periodic auctions of unclaimed items, new and used.

In recent years, Uncle Sam has auctioned such items as a sailboat that cost $20,000 for $1,777; a $686 pool table for $15; a $45 chain saw for $25, says James Senay, a Kirtland, Ohio, electrical engineer who spent two years compiling U.S. Government Surplus--A Complete Buyer's Manual (Rainbow, 120 pages, $7.95 paper).

"The consumer," he says, "seemingly is not aware of what is available." Senay's goal was to buy government goods cheaply and mark them up for resale, but work on his book has kept him from that project so far.

But Jim Hurst, property disposal officer at Ft. Belvoir says he knows of retired generals "who make a living coming to buy pieces of furniture and sell them at 10, 20, 30 times their cost."

Hurst says 62 mahogany and cherrywood buffets (from the Officers Quarters at Quantico) will go up for sale April 21 at the Ft. Belvoir Property Disposal Office. "They might bring $400 to $500 in an antique store."

Government surplus is put on sale when items are no longer needed, when they are worn out, or when offices have been closed. Other federal departments have first choice, and then the goods are offered to state and local governments. What's left over goes on sale, with proceeds going to the government's general fund. Exceptions are used vehicles and business machines, which are offered directly for sale. These payments go to the agency that owned the equipment to make replacement purchases.

At the March sale at Ft. Belvoir, the sale prices, says Hurst, averaged 4.7 percent of the government's acquisition cost. The Defense Department goal is 3 cents on the dollar.

Of all surplus goods, used cars rank highest in popularity. In a period of soaring car prices and an uncertain economy, "The bottom line," says auctioneer Russ McClain of the GSA's monthly vehicle sale, "is this is the best place to buy a vehicle" (if a bidder checks it out as thoroughly as possible in advance). Also up for auction are trucks, buses, bicycles and mopeds.

At the Navy Yard auction, two employes of the Pakistani embassy each purchased a Hornet that George Walls had targeted as potential good buys. One, a 1976 4-door sedan with 43,854 miles, went for $1,400; the other, a similar model sedan with 36,596 miles sold for $1,600. Later, a Honda "Express" moped with only 745 miles--found by the Park Service near Hain's Point--brought $310. And a very clean Fuji 10-speed men's bicycle sold for $101.

Quality of the vehicles is decidedly mixed, ranging, says Jim Pollard of the GSA Surplus Sales Center, "from next to new down to the immobile." If you opt for an older, high-mileage car, remember the GSA offers no warranty. The best it can do is provide jump-service to get a balky car started at the auction's conclusion.

A nearly new car could arrive on the lot from the Department of Transportation, which has used it for testing purposes. "It may have only 25 miles on it," says Pollard.

Bidding is fast-paced, and competition can get fierce for better-quality vehicles, says McClain, especially as the day's sale progresses and fewer cars remain. "By waiting for the last car, you could pay $500 more. It's supply and demand. You can get carried away."

"We do draw excellent prices," acknowledges Pollard. "That's one of the biggest complaints."

But vehicles are only part of what the GSA puts up for sale. The day after the car auction, the GSA offered a variety of office equipment at its regular spot-bid sale at the Navy Yard. Items included a battered skid of 110 inoperable dictaphones going for scrap ($36 total), several Royal electric typewriters in good condition (about $275 each) and five prime-condition Olivetti and Smith-Corona portables ($25 to $80). This last bunch, Pollard speculates, had been carried by traveling government executives.

"We do check for condition," says Pollard, though sometimes the machines are "so outdated the Smithsonian might want to exhibit them."

In spot bidding, you write your offer on a card and drop it in the bid box, where it is kept secret. You can submit a bid up to the moment the item is called for sale. Unlike an auction, you don't have to be present to have your bid accepted.

The local GSA sales are also heavy on general electronics, computer components and medical and laboratory equipment, says Pollard, but often little else of interest to a householder. If a kitchen blender shows up, it's apt to be, he says, "from a VA hospital and is a size for 900 patients."

A good source for other consumer-type goods are the local Defense Department surplus sales held every four to six weeks at Defense Property Disposal offices in Brandywine, Md., and at Ft. Meade and Ft. Belvoir. Recent sales have included office furniture, sleeping bags, beds, vacuum cleaners, writing paper and ink, refrigerators, photographic equipment and a few vehicles.

Like GSA surplus, the items have been offered elsewhere in the government so may be in need of repair. But, says Frank DeBolt of the Brandywine office, the public sometimes can get "pretty good bargains."

In most cases, purchasers have a few days to pay and to carry away any item they have bought. You have to make your own arrangements to haul any large item.

Both the GSA and the Defense Department also hold regional or national sales, often particularly large or expensive items or those in large quantity.

For example, 750 pounds of assorted used men's and women's "utility" clothing--in "fair" condition; original cost, $7,035--became available recently in Brunswick, Maine. Or you could have bid on 900 bed mattresses--condition "poor"; original cost, $12,870--for sale at Camp Ripley in Little Falls, Minn.

This kind of surplus generally is sold by sealed-bid rather than spot-bid or auction and may require a 20 percent deposit on the amount you bid. You can write for a description catalog and bid form.

If you can, it is wise, says author Senay, to inspect the goods you are interested in, even if it means traveling to the sale. At some large sale sites, private firms offer an inspection service for a fee. Private firms also may pack your purchase and have it shipped to you.

But chances are you won't have to go out of town to find a bargain. As GSA's Pollard points out, the Washington area with its large federal presence probably has the nation's "highest concentration of surplus property."