Suppose you're trying to sell the "world's largest pearl," valued at $4.28 million. Or an 8-ton jade boulder? A flag that went to the moon? Or 200,000 bags that happen to say "Commonwealth Processing" on them? What should you do?

Well, you'd probably do what Boyd Bennett did when he tried to sell 100,000 little springs - siphon guards that fit into automobile gas tanks. (Once the gas shortage of 1974 eased, Bennett said the demand for siphon guards, even his grade-A ones made from music wire, "died overnight.")

A Dallas manufacturer who needed his warehouse space (to hold his new product, an "ecology grill" that he said can cook eight hamburgers in less than 10 minutes using only four sheets of newspaper as fuel), Bennett turned to The Wall Street Journal's miscellaneous classified ad column.

The miscellaneous column is, in one sense, what one might expect in the way of uncategorizable esoterica in a publication that prides itself on reaching the learned, the powerful and the affluent. It can be a playground for the rich, tempting, say, a baron of industry to pluck an "original by the great masters . . . High quality investment art that will give pleasure and prestige as well as capital appreciation."

The Journal enjoys a unique status as the only national daily and has a circulation of 1.5 million, second in the country. Its subscribers are highly educated (83 per cent attended college) and highly affleunt (average household income is $43,634, with extraordinary percentages owning such - appings of wealth as art and antiques, seasonal homes and undeveloped land.

Often, though, miscellaneous sits in bold-faced contrast to the rest of The Journal. While The Journal concentrates on complex an weighty business issues - gigantic conglomerates, mammoth stock transactions - the spunky miscellaneous pops up with a distinctly human appeal, with homey blurbs of supply and demand that provide a glimpse of individuals and their concerns.

The miscellaneous is the market-place in all its peculiar, seemingly boundless variety.

There, one can find such quaint items as a "pristine treasure chest of exquisite 16th-century Belgian Congo ivory armlets." Or a $30.000 Czechoslovakian viola, cello and bass collection. A 317-karat black opal gem said to be appraised at more than $1 million. For $10,000, an early 19th-century bill of sale for a slave. A $12,000 Somali leopard coat. An 8-foot bronze cannon dated 1954. A deep-sea anchor with 75 feet of chains.

You say you want quantity? How about 75,000 pet rocks, 22,000 sets of inflatable swim trainers, 130 70-ton open-top hopper cars, 6 million pounds of green coffee, all the camel coal ("The Hottest Thing in Fireplaces") you want from Frnaklin Furman of Columbus, Ohio, or 300 "swing bikes" with rear wheels that steer "simultaneously with the front for stunts and turns never before possible."

You want presidential letters? Bruce Gimelson of Bucks County, Pa., is asking $200,000 for a collection of letters written by every President from Washington to Nixon with the exceptions of Pierce and Arthur Gimelson said it's not that he hasn't owned letters by Pierce and Arthur, it's that "They're hard to find with good content worthy of the collection. They hardly said much, those two guys."

By contrast, Gimelson points with pride to such letters as Polk's assessment of his election possibilities ("It's very long, for Polk especially.") ("He thought he had a pretty good shot.") and Washington's business sense. "This letter," said Gimelson, "was written to Gen. Spottswood advising him how to invest his money. Essentially it was to buy land in Virginia. Washington probably had it in mind to sell him a few parcels."

Letters of another sort, 90 of them, written by Jeanette MacDonald to a Milwaukee businessman and found in that man's trash can are being sold by Ronald Balistreri of Milwaukee, who found the letters many years ago while painting the man's house. "I don't know what I want for them. I don't know what they're worth," said Balistreri.

Unlike balistreri, William Livesay of Coral Springs, Fla., has a good idea what he wants for a Goya painting that he said has been appraised by Lloyds of London for $175,000. He'll sell or trade it - if he can "trade up." He's done a lot of trading, beginning with a restaurant he owned in Del Ray Beach called the Purple Pickle.

Livesay swapped the Purple Pickle, a 1953 Bentley, and some Picasso lithographs for 15 acres in the British West Indies. "I wanted to get the hell out of the restaurant business," he said. "Then I wanted to get my equity back in the country." So he traded the land for the Goya. he said the painting - "untitled, from his disasters of war series" - has been "sitting in my living room" for six months with alarm systems attached. He's ready to move it and believes he has a very marketable commodiyt.

Joseph W. Valiant of Bedford, Va., should have been so fortunate. The president of Commonwealth Processing Corp., a meat-processing company, Valiant got stuck with 200,000 bags when "the government ran over top of us with its big political steamroller" and closed down the business in a controversy involving meat import quotas.

Valiant said the bags had the name of the company on them as well as "beef chunks, keep frozen, USDA inspected, and all that good stuff. I sold them for three cents on the dollar," he added, wistfully. Apparently, you can't beat a plain bag.

Similarly, why would anyone want wood shavings if they weren't nice and dry? Irving Hibbard's chips are "They're just little scraps of wood," said the general manager of a Portland, Maine, lumber company, somewhat modestly. But, viola, they were just what Calvin Beede of Lowell, Mass., was looking for.

Beede plans to grind much to stawdust, his business being shaving and sawdust sales. "My father peddled wood for stoves," said Beede. "Then we got into sawdust and, gradually, more and more sawdust." He said there's more demand for his products in winter, when, for example, the animals are back in the barns and need bedding; but the time to stock up is now.

July or not, it's also the time for a Dallas company to move out its ice scrapers.

And it's the time when a college professor can get out from his classroom and do some manual labor. This is what a certain academician decided to do when he joined on at Toledo Salvage - only to have a bulldozer drop a railroad tie on his foot. While limping, he sprained his other ankle, and now is laid up.

"It's been a real experience for him," said Mrs. Charles J. Capazo, wife of the owner of Toledo Salvage, who related the story in a concerned aside to a query concerning the company's offering of railroad rail, 23 miles of it. She said inquiries had been made by a Houston firm considering using it in Mexico and a Canadian company interested in it for a Central America project. Right now the rail is near Cement City, Mich.

Products offered in The Journal's miscellaneous column can end up almost anywhere, a result of the paper's worldwide readership. A Florida man, for example, who is selling Canadian top soil said a prospective buyer in in Egypt. Audie Reginald, who said he is Audie Murphy's godson, said he has gotten calls from all over the country in response to his ad for two of Murphy's pistols, though he's leaning toward keeping them.

It's not so much the number of calls but the "quality" of them, said Livesay, the man with the Goya. "With real estate ads." he said. "I might get 50 calls from an ad in my local paper and no buyers. With The Journal, I might get three calls and one really meaty one."

Though the price for a Journal ad can be imposing ($85.35 minimum in the national edition and less for any one of four separate editions, such as $32.60 in the East), the results can be gratifying. Now, as for those 100,000 gas siphon springs . . .

Boyd Bennett said he is moving out 20,000 of them to a Canadian gas tank manufacturer. A one-time singer who wrote and recorded a 3.5 million-copy seller called "Seventeen" in the 1960s and sold 4.5 million gas siphons during the 1974 crunch, Bennett predicted new success for his ecology grill and companion model "Burger Bucket." Just wait.