"It's a small world and it's getting smaller," said Charles M. Seilheimer, but fortunately for the 35-year-old president of Sotheby Parke Bernet's real estate arm, that shrinking globe has an expanding number of people who can buy million dollar estates.

Sotheby and Seilheimer are in the monetarily merry new business of matching million dollar properties - and even a few little $200,000 affairs - with millionaire purchasers on an international scale.

Their clients, after 10 months of operation, read like a social register of capitalism: Henry Ford, Henry J. Heinz II, Mrs. Hugh D. Auchincloss, Sanford Simmons of Simmons Mattress, Braniff Airways chairman Harding Lawrence, John H. Wilkins of Wilkins Coffte.

"The rich always have people preying on them and trying to take advantage of them," Seilheimer said, "and the Europeans especially are very reluctant to deal with an unknown broker."

Sotheby, the 232-year-old auction firm of international fame, is a name with which the wealthy feel comfortable. In December, the firm was circulating only four of its elaborate, full-color brochures on available property. Now it has about 50 printed up on property in Virginia, Delaware, New York, Rhode Island, Florida, Louisiana, New Mexico and Vermont. Total sales have amounted to about $14 million, Seilheimer said.

The estates are put up for sale for many reasons, but death, old age and divorce and prominent causes.

Carter Hall in Clarke County, which Seilheimer describes as the finest house to come up for sale in Virginia in 10 years, is on the market to settle the estate of Mr. and Mrs. Frank Christopher who bought the house in 1947.

Another Virginia home, Corbin Hall on the Eastern Shore, is being sold because its owners are getting older, have other houses in Illinois and Florida and want to slow down a little.

Several houses on exclusive Fishers Island in New York are for sale because the owners are growing old, have other estates and town houses and no heirs who want the places.

Two Florida houses are up because of divorces. "A house is personal and it is hard to stay after a divorce," Seilheimer observed.

The houses may get listed with Sotheby because the owner wants to auction off the contents, or may have been buying art and furniture through Sotheby for years. Perhaps the local real estate broker may contact the auction firm.

Sotheby's takes on the job under several conditions, Seilheimer said. The commission is generally 10 per cent, to be divided with local brokers if they are involved and the company asks that it be given two years to make a sale.

"We are blase about talking of million dollar properties, but purchasers can be a few and far between. From the time we send a brochure, get an indication of interest and show the house, say, to a European, it may be six months. Then it may take another six months to complete a contract.

"They are not buying because they need the house, and the sellers are not under the sort of pressure that comes from being transferred and having to sell in two months," Seilheimer said.

A third thing that Sotheby requires is agreement with the owner on a marketing program and its cost. For a $250,000 house that might be $4,000, but for a $3 million to $5 million property, the cost would be in the $18,000 range. The owner must put up a check for that amount and Sotheby uses the money for brochures, mailings and a public relations campaign to let the world know the house is for sale.

When a sale is made, the marketing money is deducted from the 10 per cent commission. If there is no sale, the cost is absorbed by the owner.

"Today there are thousands and thousands of people who can afford a property above $250,000," Seilheimer said, "You can collect for both pleasthe same way."

"Real estate is like art," Seilheimer said. "You can collect for both pleasure and profit. The stock market the only pleasure comes from the profit."

This similarity is Sotheby's ace in the hole. Its art representative are in most of the world's major cities and they deal on a regular basis with many of the world's wealthiest people.

The result is twofold. First, a considerable number of rich people have dealt with Sotheby routinely over the years and, in part due to a high-powered public relations effort, thousands of other people are impressed by the cachet of the name.

And second, Sotheby has developed mailings list of art patrons that can and have been converted into mailing lists of potential real estate buyers. Sotheby can drop its glowing brochures on the doorsteps of 80,000 wealthy people around the world.

An example of the impact of name and access is "Hammersmith Farm," the Newport estate of Jacqueline Kennedy Onassis' mother. It has been available for sale for six years," Seilheimer said. "We listed it in July and we already have three contracts with checks attached."

Sotheby's buyers so far have been about half American and half foreign, with Germans being the dominant European group.

The Americans are "as varied as the houses they buy," Seilheimer said. The Europeans are looking for farming potential, a place to live and development potential in that order, he said.

They are looking for long-term investments over 20 to 40 years and consider this the safest economy in the world, Seilheimer said. They also often cannot believe the prices are so low here. Corbin Hall, for instance, at $825,000 includes a magnificent house built before 1787, a half-mile of waterfront and 470 acres - less than $1,800 an acre.

Sotheby has concentrated heavily on the Virginians hunt country, partially because Seilheimer, who lives near Warrentom when he is not in New York, is familiar with it and partially because Europeans immediately feel comfortable there, Seilheimer said.

"It looks like parts of Germany and Britain and the lifestyle is staid and conservative. Europeans are not turning their backs on their own countries but trying to perpetuate what they have there," Seilheimer said.

Seilheimer said he expects to open Sotheby realty offices in Chicago, Houston and Los Angeles in the next 18 months and is looking for a full-time broker to handle the metropolitan Washington area.

"At this point we're trading on a very good name. these are not one-shot clients. They may be art clients who buy works worth $500,000 a year through us. We can't have them come in and get turned off on a real estate transaction," he said.

"Being well known is a two-edged sword. If you aren't doing your job everybody knows about it," Seilheimer said. CAPTION: Picture 1, Flying staircase is graceful link between floors at Carter Hall. Sotheby Parke Bernet International Realty Corporation; Picture 2, The Carter Hall estate is pictured on a brochure advertising it for sale (for $2 million).; Picture 3, CHARLES H. SEILHELMER JR. . . . president of firm