Much like a major department store, Uncle Sam has a bargain basement where oldies -- and often goodies -- are there for the buying. Known as the GSA Surplus Sales Center, this federal flea market is the last bureaucratic pit stop for countless typewriters, desks, and other governmental has-beens.

At Building 197 at the Washington Navy Yard, a former turret factory and the cavernous home of the surplus sales center, the very innards of American government are for sale. Row after row of somewhat gleaming merchandise await new ownership, your new ownership if you choose.

The merchandise sold through GSA is "excess" but not necessarily junk, according to William L. Tesh, branch manager at the sales center. For example, a government agency replaced 10 adding machines from the Truman era with more efficient electronic calculators. If the machines cannot be used by another federal office or are not claimed under a state donation program, GSA can sell them through the sales center -- whether they work or not.

Like any good merchant, GSA does what it can to keep the customers happy. Free parking adjoins the center. An extremely helpful staff guides buyers through the simple purchasing process. GSA does all the paperwork. While GSA won't accept credit cards, checks are welcome and possibly preferred.

Knowing how the game works is the secret of getting a good deal from GSA. First, Uncle Sam looks only for the best offer, not who makes it. Dealers do not have an inside track on merchandise. Second, everything is sold "as is." GSA makes no guarantees. Third, while many items are cheap, the bidding process assures that nothing of quality is free or close to it.

There are three types of sales at the center. Office equipment sales, including desks and filing cabinets, are held bi-weekly on Thursday at 10 a.m. Motor vehicles are sold the second Wednesday of each month. Miscellaneous items are sold on an irregular basis and such sales are announced through public mailings.

Since everything is sold as is, it makes sense to inspect items before bidding. Office equipment, for example, can be seen daily between 8:30 a.m. and 3:30 p.m. As the stock of items is always growing, the best time to inspect this merchandise is a day or two before the sale. Motor vehicles, a category that includes just about anything with wheels and an engine, can only be seen the day before bidding.

The goods on the floor at any time may include dozens of IBM typewriters [largely modesl C and D], plus adding machines, dictating equipment, filing cabinets, desks, automatic pens, tables, and ancient wooden file holders. Most of the equipment items are sold individually but often goods are grouped. At one recent sale, for example, 21 adding machines and calculators were sold for $11.12.

A section of the floor is set aside for "miscellaneous" items. Complex scientific instruments lie next to an ancient drill press, a huge machine so old that it is run by leather pulleys. A printing press and dental equipment share another row.

Once the stock has been examined, the next task is to figure out how much to bid. GSA maintains a list of past sale prices and so obtaining a ballpark bidding price is greatly simplified.

While vehicles and large miscellaneous items are sold by live auction, the office equipment and most miscellaneous goods are sold through a spot bidding system. It works like this: A bidder registers with the office and is given an identification number. At the sale each item is announced in turn. Interested buyers then pass up post card-like forms which show their bids. The highest bid wins.

You don't have to be at the auction to buy. Each sale item is numbered and a sealed box is nearby. To make a bid, individuals need only to leave a card in the proper box. The boxed bids are then compared with other offers at the sale.

While GSA merchandise is cheap in dollar terms, it may or may not be a good buy. Recent sales have included an IBM typewriter with a 13-inch carriage for $132.18, a 1973 Plymouth for $157, 128 wooden file holders for $80, and metal desks for $5.88 and $16.91 each.

However, these figures do not indicate the condition of the items involved. Most of the goods sold need some repairing, cleaning, or adjustment.In some cases various items are good only for parts or scrap. But for the purchaser who heeds the admonition, "buyer beware," the surplus center is one of the better shows in town.