For sale: John Wayne yacht, mastodon tusks

You've got a $6 million island you want to sell. Or maybe a $20,000 classic Ford T-Bird. Or a mastodon tusk. You know an advertisement in the local penny-saver isn't likely to reach the buyer you're looking for. And you don't want to pay commission to a broker. So what's the alternative?

James Hall will tell you it's his Los Angeles-based company, The Investment Matchmaker, a nationwide, computerized matchmaking service for buyers and sellers of ''extravagant valuables.'' And he means extravagant.

That $6 million island, for example, located off New York's Long Island, is among his current listings. So is John Wayne's $2.5 million yacht. There's even a lock of George Washington's hair (complete with a letter of documentation) listed at $150,000.

What sets Mr. Hall apart from classified ads and investment brokers is the service he offers, which is loosely modeled after the computer dating concept. For $400 to $1,000, depending on the advertising package, the seller - who can call in on a toll-free line - gets a computer readout listing the names and phone numbers of buyers across the country who are specifically interested in what the seller is offering. (Buyers can get a list of sellers for free, a list also tailored by the computer for their specific needs).

In addition the seller's item is advertised in entertainment industry publications - some 50 percent of The Investment Matchmaker's business comes from the entertainment industry - and also is sent out on weekly mailing lists to company clients.

Unlike brokers, who may charge a 6 to 10 percent commission, Hall makes nothing from the sale of an item. And unlike classified ads - which can cost as much as $437.50 a day for a one-inch column in the Wall Street Journal - Hall's listing fee is good for as long as it takes to make the sale.

During the seven years he's been in business, he says, only one-half of 1 percent of his sellers have asked for their money back. According to the latest records at the Los Angeles Better Business Bureau, no complaints have been filed against the firm in the past three years.

One customer who says he's satisifed is Dave Lowell Jr. of Clearwater, Fla. After trying for nearly six months to sell his '74 Porsche Carrera through traditional means, including advertisements in the Wall Street Journal, he tried the Investment Matchmaker. Within 10 days the car was sold.

''I'm very pleased with the business,'' says Mr. Lowell, so pleased, he says, that early this month he listed another car with the company - a '69 E-type Jaguar Coupe.

''We'll be in touch nationwide with people you never knew existed,'' says company vice-president Michael Greenwald. He likes to cite the case of an Oregon man interested in selling a 20,000- to 40,000-year-old mastodon tusk, valued at the tax credit he could get by donating the tusk to a museum, but didn't know where to find buyers. He called The Investment Matchmaker, and though the tusk hasn't sold yet, Mr. Greenwald boasts that his company located the only fossil dealer in the country, who now is searching out interested buyers.

Currently, he says, the company's computer has on its memory banks the names of 2,500 sellers and 5,000 buyers. Most listings are likely to appeal only to the extremely wealthy investor, but not all are as exotic as a private island or a mastodon tusk. In fact, says Greenwald, 80 percent of the company's business centers on more mundane sales like ski resort condominiums, beachfront homes, and bowling alleys.

No sales are guaranteed, just as no newspaper guarantees sales from its classified-ads sections. Still, says Greenwald, the firm tries to guarantee that buyers on its computer lists will be seriously interested by screening callers. A prospective buyer is asked whether he or she has money in hand, is willing to buy, and is willing to travel to look at the property listed.

''We don't want somebody who says they'll be able to buy after they've sold their house,'' he explains. ''What we want is somebody who's always looking.''