Army Col. Robert Bartos and his wife knew exactly what they wanted when they showed up on one of their frequent weekend visits to Laws auction center in Manassas. Their quest: a 19th-century painting of a dreamy maiden welcoming a curly-haired cupid through the window.

They have, says Mrs. Bartos, furnished their Alexandria home with mostly older pieces picked up at Washington area auction houses. The large Romanic canvas "fits" their decor.

The Bartoses had seen the framed work at an earlier auction when it went unsold after failing to get a minimum price. This time auctioneer Sonny Laws opened at $350. With no takers, he immediately dropped the price. Brief, but fast-paced bidding built it right back up again.

In less than a minute, the Bartoses bought the unsigned, untitled painting for $375, "about $100 more than we expected to pay." Nevertheless, they are convinced they made a good buy.

In the first place, says Col. Bartos, they like it. "The cupid looks very good." And, he points out, a previous owner went to substantial expense at one time to give it a new back. He figures they wouldn't have bothered unless it was worth the money.

Minutes later, Charles W. Earley of Camp Springs, Md., made what he concedes is probably a worthless buy -- 10 cases of empty glass cigar holders, in which single cigars were once sold.

"I don't know," shrugs Earley, "what I'm going to do with them." His $5 bid -- the only one -- was accepted while his wife was away at the snack counter.

Earley knows better, he confesses, after taking some ribbing from some in the crowd of 80. An antique clock fancier, he's a regular at auctions to add to his collection of more than 50. "It was just a gag. I thought somebody would offer $6. I should have bid $1."

Back with the coffee, his wife takes the purchase in stride. "Oh, no," she laughs. "It's probably my Christmas present."

Win or lose, auctions are big business. Mulitmillion-dollar Old Masters make front-page news, but across the land everything from tobacco and hogs to actor John Wayne's "True Grit" eye patch (which reportedly drew $15,000 for charity) to $1-to-$10 household items regularly go on the auction block.

For many Americans, auctions serve two purposes. They can be an inexpensive outing in which the auctioneer provides the entertainment while you try your hand at what Auction! (Sterling) guidebook author William C. Ketchum calls "the serious game of legalized gambling."

In Washington, a lunchtime crowd of hamburger-chomping office workers turns out for the all-day Tuesday sales at Adam Weschler & Sons. Says appraiser William Weschler Jr.: "There's a McDonald's right next to us. They come over here and listen."

And for the discriminating bargain-hunter in this era of high inflation, they can be "a fantastic source of fine-quality furniture, jewelry or housewares" at below retail prices, say longtime Washington auction-goers Andrea Lubershane and Erik Kanin, publishers of The Inflation Fighter's Guide to Best Buys in the Washington Metro Area (Andrik Associates, 500 pages, $6.95).

For example:

Six years ago in Strasburg, Va., Kanin bought a Civil War-era campaign chair for $2.50. "An incredible buy."

At the same auction, Lubershane picked up a huge Italian baroque-style mirror with beveled edges for $4. "It weighed a ton," but she sold it later for $55.

A friend who is good at furniture repair, they say, bought the pieces to "11 real Chippendale chairs" for her all-colonial house for $400 and restored them. "You can't buy one chair for $400."

Lubershane's parents have furnished "their whole house with antiques from auctions," including "a 200-year-old French bed for about $45 -- a fantastic bargain."

Kanin's $18 bid won him an old solid-oak carpenter's chest with brass handles. "Five minutes after I bought it, somebody came up and offered me $120 for it."

(Their auction guide is one chapter in a book that lists bargain clothing stores, appliance shops, tire and auto part outlets, restaurants, theaters and other firms in Washington and Baltimore.)

Top-quality antiques usually are advertised at "catalogue" sales, where items are described in a booklet. There is often an admission fee. Less valuable household items are offered at "general" sales, usually without the catalogue or entrance charge. Weschler and C. G. Sloan & Co. in downtown Washington frequently hold both types.

At a recent Weschler auction, bidders in a standing crowd of 40 quickly snapped up a used chest of drawers for $140; a mattress, $40; a white wrought-iron garden settee, $90; a drop-leaf mahogany dining-room table, $60; and two well-used color TV sets, $80 and $50.

In addition, note the pair in their best-buys guide, a number of federal and local government agencies hold special sales of surplus, confiscated or used equipment such as office furniture, cars, trucks and even boats. They favor for themselves the folksier (and often cheaper) country and farm sales.

"A lot of old people in the country still have a lot of stuff," claims Lubershane, handed down in their families.

Pre-'40s objects, especially table knickknacks going for $10 to $20, appeared to dominate the inventory one Saturday morning at Laws June general sale. A green Depression-glass orange-juice squeezer brought $8; a hand-blown glass cruet set, $12; two moustache cups, $11. Brass beds and other furniture pieces, some dating from the mid-1800s, were to be auctioned the next day.

But, warn Lubershane and Kanin, buying at an auction can be risky if you don't know what you're doing. Lubershane recalls bidding $50 over my limit for a diamond and sapphire bracelet" she was convinced was a great deal. Later, she learned she could have got it from discounters W. Bell & Co. for about the same price.

Auction houses put items on exhibition a few days or a few hours before the sale. This is your chance to inspect them closely.

"Bring a magnifying glass," says Kanin, to look for cracks or chips in crystal or scratches in furniture; goods are sold "as is" with returns usually not accepted. Check to see that the table you are interested in is not a less-valuable "marriage" of the pedestal from one piece and the top from another.

If you're looking for a particular type of item, research it before you leave home. One friend familiar with a style of desk that usually comes with a secret compartment found one at a New England auction. He poked open the undiscovered hideaway and spotted "old gold coins and a diamond." He quickly reclosed the compartment and bid until he got the desk.

"Go to some retail outlet such as Woodies or Hechts," suggests Weschler, "to see what they're getting for a piece. Don't get caught in the heat" of the bidding. "You can overpay."

Good buys these days, he says, can be found in used china because retail prices are so high.

"We sell partial sets you can get fairly reasonably. If you see it's an active pattern, you can buy the retail pieces to complete it." Twenty pieces of Limoge in a green and gold design recently sold for $40. Upcoming is an auction of the now-defunct Auto-Train's office furniture.

In Washington, about half the people in an auction crowd will be antique dealers (in New York, the percentage is even higher) who are experienced in the buying game. But, says Weschler, "There's nothing scary about them. The dealers have got to bid on a piece just like you do."

And, since they may have to mark up the piece 40 percent or more to make a profit, they may drop out of the bidding. "If you get a piece in competition with a dealer, you got a bargain," says Weschler.

"Never plunge into an auction immediately," advises Sloan auctioneer Stephanie Kenyon. "The custom is, the auctioneer will ask a higher price than he expects to get. 'Who offers me $500?' If they're no hands, he'll ask, 'Then, who'll offer me $200?' because he knows its worth $400."

But, she adds, if you have your heart set on a certain vase, "You have to be alert. You have to make decisions quickly. An auctioneer won't wait if you can't make up your mind." At Sloan's general sales, she says, you can get good buys on used "household goods and occasionally appliances and color TVs."

When you're calculationg costs, warn Lubershane and Kanin, don't forget to include the price of refinishing or repairing, sales tax and hauling if it's bulky. Also, Sloan and Weschler (but not Laws) charge a 10 percent commission to both the seller and the buyer. The $100 you bid for a nightstand may become less of a deal at a total price of $116.60.

As for the possibility of scratching your nose absentmindedly and having the auctioneer take that as a final $60 on a stuffed buffalo head, don't worry. For one thing, many auction houses give you a numbered card which you raise to bid. (This is not the case, however, at Weschler's, where a nod or a raised finger can do the job.)

Or, says Weschler, just tell the auctioneer, "I'm waving to my mother." He'll just go back to the $50 bid. We're not going to hold you to it."