Sanford and Patricia Smith's involvement with American antiques began 20 years ago at rural New England auctions. It has grown to include a gallery on Madison Avenue, management of two important New York antiques shows, and raising their family in a 19th-century landmark town house in Manhattan.
''In the mid-'60s we knew nothing about antiques,'' Mr. Smith recalls, ''but when we went to visit my wife's parents in Connecticut, we would go out to the local Grange hall on Friday nights to country auctions. We bought boxes of old books for 50 cents, boxes of bric-a-brac for 75 cents, and old picture frames for a quarter. For $10 we would go home with a whole carload of stuff to stash away in my mother-in-law's basement. Other than the local movie, these country sales became our chief source of amusement.''
In the spring of 1965, the couple rented a stall for $15 at the 26th Street Flea Market in Manhattan, fetched their Grange-auction finds from Connecticut, and, as a lark, offered them for sale to the public.
''Because we were such novices, we began with old Life magazines, big-little books, and movie star posters,'' Mr. Smith recalls.
''Our first Sunday we grossed $150, made a $50 profit, and had a wonderful time. We were at the flea market every weekend after that, buying and selling and observing. We had so much to learn. But it all seemed a great adventure to us, and we soon felt we had discovered our real vocation. As time went on we were able to nose out and offer better and better quality and to exhibit in other shows.''
It was in 1969 that the Smiths extended their love of old things to include a vintage house. That year, they purchased their 1849 town house in the lower-Manhattan Chelsea area for $65,000 and spent an additional $70,000 on a complete interior renovation.
The exterior remained intact because it is one of 12 houses designed and built by Philo Bebe on what was the original farmstead of Clement Clark Moore. The house has been declared both a city and state landmark and is listed in the National Register of Historic Places.
The couple's remodeling efforts divided the house into an upper rental apartment (''we needed that extra income in order to carry the house'') and a lower apartment to house them and their three now-teen-age sons, Colin, Jared, and Ian.
Mrs. Smith planned the big heart-of-the-house kitchen as a place for much of the family action - cooking, dining, entertaining, living. Here the family makes good use of her own special collections of Ashburton Sandwich glass and spongeware and spatterware pottery, in a homey setting that includes a cupboard from the Massachusetts North Shore, made about 1790; a Pennsylvania Dutch dining table made in 1860; a cigar-store Indian; and a collection of three different sets of New England spindle Windsor dining chairs.
Over the years, the Smiths have filled their town house with an amiable assortment of 18th-, 19th-, and 20th-century furnishings that somehow fit compatibly together. The melange includes Western art, Navajo rugs, a Queen Anne wing chair, 1920s and '30s architect-designed ''modern'' hand-painted trunks, Harry Jackson bronzes, Shaker furniture, Grenfell rugs, 19th-century weather vanes, marine paintings, and military miniatures.
The Smiths recently completed their secondary renovation, reclaiming the top two floors for their own use and building two efficiency studio apartments into the lower, or ground floor. These small apartments, each with kitchenette and bath, were built with foresight for use by family members only, including sons, parents, or parents-in-law.
The year 1979 was a banner one for the couple. They opened the Smith Gallery at 1045 Madison Avenue, which Patricia Smith manages, specializing in American marine paintings, western, American Indian, and other American folk art.
That year, also, Mr. Smith was invited by Dr. Robert Bishop of the Museum of American Folk Art to become the first producer-manager of the all-American Fall Antiques Show at the Pier, a project intended to help raise money for the museum. The success of this show launched a new aspect of his career - show management. He is now putting together the sixth Fall Antiques Show, which will take place Oct. 25-28 at Pier 90 on the Hudson River. This spring he also launched what will be the annual Armory International Antiques and Fine Art Exposition.
''If I have a success formula,'' he says, ''it would be wide diversity and top quality.'' He sees his shows as filling a void between the ''decorator-elegant on the one hand and the indoor-collectible shows on the other.''
He has also dispensed with dating limitations, which means that this fall he will include 1940s and '50s designer furniture and accessories in his mix. He has anticipated the trend of public interest in the past by being and was among the first to feature American country furniture and Bauhaus School designs and to emphasize the importance of American Indian art.