You just don't know what's available until you browse through Previews, Fine Homes, Homes International, and Unique Homes:

* A four-bedroom New Hampshire farm house with an attached six-plane hangar and an airstrip--$595,000.

* A 15th century Norman Castle--Ballyportry Castle in County Clare, Ireland--complete with a master bath tub that is "a handsome trough from Lady Gregory's Estate"--$550,000.

* A small Japanese castle built in 1689 on the island of Shikoku that was carefully dismantled in 1978 and stored in crates. The purchase price of $1.5 million includes reconstruction by a team of Japanese artisans on a site of the purchaser's choosing; once rebuilt, a Shinto priest will be flown to the site to bless the land and purify the structure.

* A two-story Georgian-style mansion--with elevator--in Palm Beach, Florida--$3.5 million.

* Capo di Monte, a Mediterrean-style estate on 4 1/2 acres in Bel Air, Calif. complete with "a cool, columned loggia, classic pool and serene terrace"--$11 million.

The properties are featured with hundreds of others in thick-cover, glossy, coffee-table magazines filled with mouth-watering pictures of glorious villas, cottages and castles; breath-taking views of oceans, bays, rivers, mountains and glittering cities at nights; designer interiors; and detailed descriptions of the amenities of the main residence and servants quarters. Practical details often are included in the descriptions, such as the property's air-conditioned wine celler, cedar closets, telephone outlets.

For those who want to own something that belonged to "somebody," available right now are a fully furnished villa in Marrakech once owned by the King of Morocco ($1.5 million), John DeLorean's 48-acre estate in Pauma Valley, Calif. ($3.7 million), Andy Williams' "Two Trees Estate" in Aiken, S.C., Gene Hackman's estate in Montecito, Calif. ($12 million), the Hawaiian home built by Clare Boothe Luce and the late Henry R. Luce ($4.35 million) and Helen Reddy's Brentwood, Calif., home ($3.6 million).

The glossy guides are used to develop exposure for properties that won't sell in the local market, according to Bruce Wennerstrom, president and publisher of Homes International Corp., a bimonthly magazine, and president of Previews Inc., the publisher of a nearly-400-page annual guide to real estate throughout the world. Both are subsidiaries of Coldwell Banker, which is owned by Sears, Roebuck & Co.

"Previews is an annual catalogue that serves the same purpose for us that Sears' catalogue serves for it," Wennerstrom said. "Sears isn't in the business of publishing catalogues; it is in the business of selling merchandise. We're in the business of selling expensive properties; it's one of the tools we use."

It appears to work. As an example, Wennerstrom cited a lodge in Vermont they sold several years ago for $160,000, "a low price for the sort of things we handle." The house had been on the market for three years, listed with the area's three brokers. "Everybody for 50 miles around knew the house was for sale," he said. They thought someone would drive through, decide he or she wanted to live there, and find the house available.

After the house was advertised in Homes International, it promptly sold to an oil company executive. He was reading the magazine in Eastern Airlines' Ionosphere Club, Wennerstrom said. "He liked the photo of the house and what he read. To him, $160,000 was not a problem; it was a question of national exposure finding a buyer for something that wouldn't sell in the local market."

The $18-a-copy Previews, an apparent granddaddy of the genre, was started in 1935 and has a circulation today of about 22,000. It is sent to the chief executive officers and real estate officers of all Fortune 1000 companies, trust officers of major banks, to first-class lounges of airlines, and to plastic surgeons in affluent areas, "on the basis that people who have cosmetic surgery have high disposable income," Wennerstrom said. It is also on selected newsstands, those believed to cater to affluent areas, and quality hotels. "It doesn't serve our purpose if people who can't afford it buy it," he said.

An outside study of readers indicated that 35.6 percent of Previews' paid subscribers have a net worth over $1 million; 10.6 percent are chairmen of the boards of corporations; 14.2 percent are either president or chief executive officer. The average household income of subscribers of Homes International in 1981 was $115,145 a year, and 23.4 percent had a net worth over $1 million.

Homes International, started in 1970, is aboard 15 major airlines and sold on newsstands throughout the country, in Canada, Australia and Europe. Each issue costs $3.95. Total circulation is near 100,000 with 31,000 sent free to the airlines. If the magazine goes to a home, the readership per copy is about two or three people, while "dozens of people see a copy that goes aboard an airline," Wennerstrom noted. As a result, they estimate the "readership" at 3 million.

Previews is a real estate company marketing properties listed with it--and shown through 15,000 servicing brokers--but Homes International is run as a magazine in which Previews buys advertising, just as other real estate firms do.

There is no minimum price per property required for any of the magazines. The least expensive property Wennerstrom included that he remembers was $13,000 for a picturesque stone mill in the south of France available about six years ago. One of the most expensive available now is "El Destino," the 8,320-acre plantation and home of Col. James Gadsden, the U.S. diplomat who arranged the purchase of part of the Southwest from Mexico in 1853. The pricetag: $18 million.

Although some of the publications, like Unique Homes, include only residential property, both Coldwell Banker publications include nonresidential properties. The 1983/84 Previews includes such properties as an island in Bermuda, a cattle ranch and farm in Argentina, a fruit plantation in Brazil, a fishing ranch in Montana, four industrial/commercial buildings in Arkansas, a hotel in Honolulu, a theater in New York, a 70-acre summer camp in Pennsylvania, a college campus in New York, hotels in the South Pacific.

Unique Homes is the only independently owned real estate magazine not connected with a real estate firm, according to publisher Jeff Hammond. The magazine was founded in 1973 by David and Andrea Fritzen and purchased by Ziff-Davis Publishing Co., the nation's largest publisher of special-interest magazines.

Any company can buy a page of advertising but its ad must fit into the magazine's fixed format style, Hammond said. Generally, Unique Homes' staffers take the pictures and write the copy from specs provided by the companies. Houses in the magazine range from $200,000 to $15 million, he noted, with the average price of a house about $1 million in the last three issues.

The magazine also accepts general consumer advertisers from companies "trying to reach this very wealthy audience," he said. The ads in the latest issue include Porsche, Remy Martin Cognac, Jaguar, Alfa Romeo, Taittinger Champagne, Ferrari, Cadillac Eldorado. "The reason is that 31 percent of our paid readers have a net worth in excess of $1 million," he added.

Published six times a year, the magazine has 75,000 paid circulation, counting subscriptions and newsstand sales; selling for $6.95 an issue, it is placed in newsstands "where we know wealthy people go," Hammond noted. "We don't want casual lookers reading the magazine," he said. "Our magazine is designed solely for people who are interested in buying real estate." It must be working: "The magazine sells real estate every month--because if it didn't, they wouldn't advertise with us again."