ON A ONE-day visit to Los Angeles to appear on a television show featuring her recently published book, "Secondhand Super Shopper" (M. Evens & Co., paperback, $7.95), Ellen Weiss squeezed in 20 minutes at the Children's Hospital Thrift Shop four blocks from the TV station.

She emerged with an old Whitman's candy box, $6; a silver-plated stuffing spoon, $9; a book of essays by Alistair Cooke to read on her return flight; an Albert Nippon dress for $16 ("a perfect fit"), and a silk chiffon tunic (Oscar de la Renta label) for $4.

Before leaving her home in Philadelphia she had looked under "Social Service Organizations" in the Los Angeles Yellow Pages ("the telephone company is very nice about supplying Yellow Page phone books for other cities"), picked out Children's Hospital because it is near the TV station, phoned to find out shop hours and to ask if it was a teaching hospital. "It is important to find a large service hospital. They have well-organized auxiliaries and the best thrift shops."

Ellen Weiss may be the world's greatest expert on secondhand shopping. She loves it and has been at it for over 35See SHOPPER, Page 3, Col. 5 SHOPPER, From Page 1 years. Her first purchases were clothes for her family: a university-professor husband and six children. Her forays into Goodwill, Salvation Army and St. Vincent de Paul stores soon led to buys in furniture, bibelots, kitchen equipment and, above all, books -- especially children's books.

She says, "Washington means culture to me because of the museums and wonderful book fairs. I always used to bring my children with me on Washington trips. They loved the museums and would tolerate secondhand book shops, although they usually wouldn't go into other secondhand shops. They sat outside in the car." Guaranteed to teach a mother how to whirl through a shop in a hurry. Her favorite is the Salvation Army Book Store, 512 First St. NW. There happens to be a regular Salvation Army Thrift Store right around the corner, into which she also darts.

But it is the book bazaars that make this city important for her, including the famous, annual Association of American Foreign Service Women's Book Fair held at the State Department, and the equally well-known Vassar Book Sale, which takes place in the historic Pension Building, 440 G. St. NW. (AAFSW and Vassar also sell fine arts, collectibles and music at these important fairs.) Weiss has often made special trips to Washington for the once-a-year Stone Ridge School Benefit Book Fair (Rockville Pike, Bethesda): "wonderful books and wonderful white elephants." And she usually finds time here to circle through a Goodwill store or two.

Weiss has no trouble shopping in strange cities.

Using the Yellow Pages and the local paper classifieds, she locates convenient sources. Once, during an hour's train layover in Cheyenne, Wyo., she hurried to a Salvation Army store only blocks from the station, and for $1.50 total bought a pair of cowboy boots ("would fit somebody in the family"), a pleasing, oval-shaped, metal bucket and a nice, floppy, stuffed animal. She put the toy and the boots in the bucket and hopped back on the train. "Much more fun and much more flavor than the dreary souvenirs in the station gift shop."

In fact, Weiss finds that visits to secondhand stores, not only in this country but in Europe, have taught her more about local customs than any number of visits to fine department stores. When in Houston, she skipped Neiman-Marcus, but came away from the Goodwill with a 25-cent first edition of the Wizard of Oz. In London she prowled through thrift shops on High Holborn Street and antique markets on Portobello Road, and had a great time at the "jumble sales."

Asked about her favorite purchase, she pointed unhesitatingly to her lapis lazuli beads. She had plucked them off a rack at a church fair a long time ago because she admired the color. It was hours before she discovered she had a valuable necklace.

But probably her best story is about the custom-made Burberry hunting suit bought for her husband at auction years ago for $3. Recently Burberry's New York store held a contest for the six oldest Burberrys, with a new Burberry as a prize for the winners. Weiss entered the hunting suit, won, and her oldest son got the new coat. When asked by a Burberry employe with an exquisite English accent how she had acquired the suit, she simply couldn't tell the truth: "My father-in-law bought it in London." She has since confessed to Burberry's. To cap things off, the Metropolitan Museum wants the suit for its fashion collection, but her husband loves wearing it and can't yet bring himself to part with it.

Weiss shares what she has learned in her entertaining book. "Sources for Secondhand Stuff" and "What's It Worth?" are covered. There are chapters on clothes, furniture and collectibles (she is against collecting for investment and explains why). She describes the difference between thrift shops and thrift stores, and tells all about flea markets, garage sales, house sales and once-a-year bazaars. The chapter dealing with auctions is one of the best. Reserve systems, what to look for at pre-sale exhibitions, the techniques of bidding in person and absentee bidding, on-site sales, country auctions and silent auctions are described in a common-sense way. And she writes, "The most important criterion for me is whether I can trust the auctioneer." She tells how to spot the untrustworthy.

The only subject not dealt with in detail is how to sell. She enjoys buying but she doesn't like to sell. She donates everything she can't use to charity.