Joel Vernick of Garrett Park, rummaging through a box at the Montgomery County Swap Meet, described himself as "an old flea market goer." His best buy, he said, was a $5 clock that he repaired and is now worth $200.

Flea markets, swap meets, thieves markets -- by any name, they are chances for one-stop shopping that have caught on with hundreds of area browsers searching for inexpensive household items, collectible glassware and books, the shopper's dream of a $5 item worth $250, and simply good, inexpensive entertainment for a weekend afternoon.

"We don't have any junk here," one seller said defensively, and perhaps there is a better way to describe old plastic margarine containers (a nickel each), a record by the Four Freshmen ($1), a "Dewey in '48" necktie ($15), a pole climbing belt ($35), a Ouija board ($4), a Depression glass bowl ($4.25) and an opened beer can (25 cents). But though the goods on sale are trash to some and treasure to others, it's clear that flea markets bring out the best of both worlds.

Organized markets, some with as many as 250 sellers, have sprung up in county and city parking garages, shopping mall parking lots and drive-ins. Some are regularly scheduled and others are one-time events, usually held on Saturdays and Sundays.

"Instead of having a yard sale, many people would rather bring it here," says Jim Bonfils, manager of The Farmers' Flea Market in Bethesda for the past eight years. "If you have your own yard sale, you have people trudging all over your yard. . . . Here, we have an accepted position and it attracts all kinds of customers."

Sellers are divided into three major categories: one-time vendors who have just moved or cleaned out a basement; those who have become so interested in collecting special items, such as glassware or linens, that they buy, sell and swap often; and professional dealers in collectibles or antiques.

Some county governments sponsor the sales as a community service and a way to organize trash collection. In Montgomery, information about the county's monthly swap meet comes from the Division of Solid Waste, whence came the original idea of sponsoring the successful swap meets and for using as landfill anything abandoned at the end of the sales.

In Prince George's County, the sales begun last year are growing steadily. Items not sold which participants want to discard are collected at the end of the day and taken to a trash dump.

According to Marion Smith, who supervises the Montgomery swap meet from a golf cart circling the floors of the county government parking lot, the sales have become so popular that vendors arrive early in the morning to line up for a choice selling location. Inside, they set up their wares in the manner of a Turkish bazaar, with items spread over their cars, clothes strung up on ropes and odds and ends arranged on tables. Sellers barter with potential customers and trade items among themselves -- hence the name swap meet. On a sunny Saturday, the atmosphere outside the garage is that of a suburban shopping mall at Christmas, complete with traffic jams, a scramble for parking spaces, and enterprising vendors hawking refreshments.

Catherine Donroe of Cheltenham says she got hooked on the Montgomery County sales and now drives an hour once a month to attend.

"This is like a yard sale shopping mall," she said as she wheeled her purchases in a baby stroller around the levels of the parking garage."My daughter sold all her toys here for $9 last month and now she has money to buy new ones. It's great entertainment for us and when we get home, it's like show-and-tell and everyone runs out to see what we've got."

Michelle Malcolm of Silver Spring, a collector of vintage clothing, said at a recent Columbia flea market: "The quality I can find in old clothes is far better than today. And it's fun even if you don't find anything. It's like a county fair."

"I'm moving soon and I want some things for a new apartment," said Joy Klingaman of Rockville, who had snapped up a drop-leaf table for $10 at a recent Farmers' Flea Market. "It's good quality stuff and it's the greatest recreation this side of Ocean City."

What's recreation for some is cash in hand for many of the vendors. They range from three families who filled up a truck with used clothing and housewares to sell from a parking space at the Rockville sale, to a New York art deco dealer at the Columbia Flea Market, to a Hyattsville woman at the Farmers' Flea Market who buys and sells glassware to supplement her Social Security checks.

Lynn and Robert Marshall of Golden Beach sell at the Prince George's County Garage Sale almost every month. It started when they got married last year and combined two households. Said Lynn Marshall, "The people where we live don't like us to have yard sales. We heard about the Prince George's County sale and started going there. We each had houses full of things so we combined everything. It was a matter of we had to go or the junk had to go, so rather than getting rid of it at one time, we stacked it up in boxes and little by little through the sales, we are digging ourselves out of the avalanche.

"For pricing, we watch the economy. . . . We go by how much it sells for new now and how much we paid and how many times we used it," she said. "We're not trying to get rich."

The Campbell, Mocarski and Townes families from Alexandria got up at 4:15 a.m. the day of the last Montgomery County Swat Meet to get a good spot. They had gathered up a truckload of children's clothing to sell.

"We had a neighborhood community garage sale where we made a few hundred dollars each," says Nancy Townes, "and it worked out so well we thought we would try to sell the rest of our things here."

Two of the families said they used to give things to the Salvation Army or Goodwill, but now would rather sell their families' outgrown clothing themselves.

The "linen lady," Mary Ann Sheffler from Waynesboro, Pa., holds court every Sunday at the Bethesda Farmers' Flea Market, where her loyal fans keep coming back for her beautifully ironed and starched linens, laces and embroidered tablecloths.

"I got started doing it in 1976 and have enjoyed it ever since," said Sheffler, who goes to estate sales to buy her linens, then spends hours washing and ironing them.

"Everything she has here is all neatly laundered and many are things you can't find in shops anymore," says Lita Colligan, a regular customer who bought all her Christmas gifts from Sheffler last year.

Virginia Holtzman, a regular Bethesda vendor, says her glassware sells well but adds that "people are getting tighter with their money." Some shoppers make insulting offers. "People will pick up something marked $5 and say '5 cents,'" she says.

At the Columbia Outdoor Flea Market, Lorraine Przywara of Essex, a rehab counselor, says the worst aspect of selling at flea markets is "the packing, hauling, moving and lifting.It's like you are constantly moving."

Her husband Ron, an engineer, says that items less than $25 are the best sellers. "We will make sales for more, but most people coming out on a Sunday are not willing to part with a lot of money."

Some tricky Maryland State Laws govern licenses and sales tax for flea markets. Briefly, sellers without a license can participate in three sales of their personal effects a year, provided the sales don't make up more than 10 percent of their total income. Otherwise a license is required. For more information on licenses call 952-3330 in Montgomery County and 279-1430 in Prince George's County.

As for collecting tax on sales at flea markets: if the things sold belong solely to you and have not been purchased for resale or received from other persons to sell, and if everythings sells for less than $1,000, it is considered a casual sale and you don't have to charge tax. For more information call the field office section of the sales tax division at 383-7319.

Here are some tips from flea market buyers and sellers:

Come early or stay late. The best items sell first. If you wait until closing, however, you can often get great buys on things sellers are sick of packing up.

Bring your own shopping bag and bring cash if possible; some sellers don't take checks.

Don't wear your Bill Blass silk dress or your Gucci loafers. It's harder to plead poverty when bargaining with vendors.

Though you'll see many signs saying "Make Me An Offer," don't make those offers insulting. You'll only antagonize the sellers.

If you're spending big money for what is supposedly an antique, try to get written documentation from the seller.

Call before you go, especially if it's cloudy. Many flea markets and swap meets are held outdoors and are cancelled when it rains. Where to Hunt for Trinkets, Treasures

Following is a list of some of the larger organized flea markets in the Maryland suburban area. Don't forget to check classified ads and local bulletin boards for smaller sales going on in your neighborhood:

The New Columbia Outdoor Flea Market, Columbia Mall parking lot, Columbia, every Sunday from 10 a.m. until 5 p.m., rain or shine. Free admission for buyers; sellers pay $30 a Sunday for a spot. Telephone 995-0118 for further information.

Gaithersburg Community Center Flea Market, sponsored by the city Department of Parks and Recreation, at 810 S. Frederick Ave., Gaithersburg. The second Saturday of every month; rain date is the following Saturday. Free admission for sellers' space is booked up until October, when the market closes. 840-1862.

Swap and Shop, sponsored by the Area Four Office of the Montgomery County Recreation Department, at the county public parking garage, corner of Fenton and Pershing streets in Silver Spring. The third Saturday of every month, 7:30 a.m. until 3 p.m., rain or shine. Admission is free; vendors pay $1 per car. 593-9670.

Monthly Garage Sale, sponsored by Prince George's County at the County Administration Building parking garage, on Governor Oden Bowie Drive in Upper Marlboro. The second Saturday of every month, 9 a.m. until 3 p.m., rain or shine. Free to the public; vendors pay $2 for two parking spaces. 952-3349.

Swap Meet, sponsored by the Montgomery County Department of Environmental Protection, at the County Office Building parking garage, Jefferson and Monroe streets in Rockville. The fourth Saturday of each month, 9 a.m. until 1 p.m., rain or shine. Browsers pay 25 cents per person; sellers pay 50 cents a car and 25 cents a person. 840-2370.

The Farmers' Flea Market, 7155 Wisconsin Ave., Bethesda. Open Sundays, April to October, 8:30 a.m. until 6 p.m. Market is outdoors and is canceled in heavy rain. Vendor rates are $15 to $25, depending on space; free general admission. 966-3303.

Big Vanilla Market, 1209 Ritchie Highway, Arnold, Md. Every Sunday, 10 a.m. until 4 p.m., rain or shine. Free admission; vendors pay $10 for a space ($8 in advance). 544-0800.

Community Flea Market, sponsored by the Hungerford-Stoneridge Civic Association in cooperation with the Rockville Department of Recreation and Parks. At Elwood Smith Community Recreation Center, Mercer and Harrington roads, Rockville. Saturday, July 11, 10 a.m. until 4 p.m.; rain date is Sunday, July 12. Sellers must live in the Hungerford-Stoneridge area; browsers admitted free. 424-8000, ext. 309.

Rhodeside Flea Market, Route 15 North and Biggsford Road, Frederick, Md. Indoor market daily; outdoor market Friday, Saturday and Sunday from 8 a.m. until 6 p.m. Vendors' fees are $12 for a weekend; Fridays are free. Browsers admitted free. 898-7502.

Burtonsville Flea Market, at the Burtonsville Shopping Center, routes 29 and 198, Burtonsville. Outdoors, every Sunday through Nov. 15.Canceled in case of heavy rain. Open 8 a.m. until 5 p.m. Admission is free; vendors pay $7. 490-0112.