His friends joke that they wouldn't buy a used car from Sidney Howard. But because of him, Washingtonians can now buy used cars directly from each other.

Just about anything that runs is on display at the Sunday Auto Exchange, a once-a-week auto flea market that Howard established last summer in a downtown Washington parking garage.

With no obligation, buyers can browse through cars parked on nine levels of the garage. To display their cars, sellers simply pay $15 and place a sheet under the windshield wiper describing the car's features and the asking price. There are no middlemen, no dealers, no fat cigars or pinky rings, and no pressure.

The exchange has proved to be "a very, very valuable service to the public," Howard said. He added, however, that sales have been "moving rather slowly."

Howard, 57, a management consultant from Bethesda, began the exchange in mid-July. It operates between noon and 5 p.m. every Sunday in a garage on the northwest corner of 18th and L Streets NW. About 40 buyers or sellers show up each week.

Besides offering sellers space and buyers a wide range of cars, the auto exchange provides information about insurance and financing, sales forms and current average prices for any make and model as listed in the National Automobile Dealers' Association handbook.

The exchange also keeps a file of index cards. On them, potential buyers can list what kind of car they are seeking.If a seller appears with such a car, and the potential buyer is not there, exchange will help buyer and seller meet.

The exchange assumes no responsibility for the condition or financing of a car.

"All we do is provide a service," said Howard, who added that he is not now, and never expects to be, making a profit from the venture.

Howard said that many of those who have turned up at the exchange so far have been women.

"Many of them are afraid to advertise," he said. "They don't want strange people calling up. And they get ripped off by a dealer."

On-site sales have been scanty so far, however, Howard said, partly because "no one has $3,000 in his pocket" and partly because many sales are completed privately later in the week.

"Often the buyer wants to think about it," Howard explained.

One seller on a recent Sunday, Deborah Duff, 24, of Arlington, said she had learned of the exchange that way and was "pleased with the whole experience. Washington needs a little more of this kind of ingenuiry," she said, as she sat on the front bumper of her unsold 1975 Ford Granada.

Many sellers at the exchange were offering big cars, and many said they chose the exchange because they had been unable to get offers they considered fair from dealers or newspaper readers.

"I advertised in the Post last week," said Tom DeWald of Arlington, who was trying to sell a 1975 Cadillac Fleetwood for $7,500. "Nobody offered me within $1,000 of what I want. No one has here, either, but if I don't get what I want out of it, I'll keep it."

"I'm sure I can find something one of these weeks," said another man, from Southeast Washington, who said he had been to the exchange fo three Sundays in a row. "This is a real good idea. It should catch on."