The Reagan administration yesterday for the first time tried to sell American products overseas the way it usually sells its policies.

Using technology to sell technology, the United States Information Agency beamed an hour-long "Worldnet" television special to 19 European countries urging buyers to attend the National Computer Conference in Las Vegas in June. The program, which was seen in Europe in mid-afternoon, had a potential audience of 4 million viewers.

People watching the program in American embassies in London, Paris, Rome, Stockholm and Bonn were able to question American computer experts taking part in the program here.

The broadcast was part of the Department of Commerce's Foreign Buyer Program, which will both help American companies market their products overseas and assist foreign buyers who visit the computer show.

Officials of the computer show and of USIA said this is the first time "Worldnet" is being used to support U.S. economic objectives. Mary Rich, chairman of the National Computer Conference, said the use of the high-technology video conference and satellite linkup is likely to give Europeans considering attending the computer show a first-hand look at the new U.S. technologies that will be open to them at the computer conference.

"It had to be impressive," she said. "We really put together this particular episode of "Worldnet" to sell the Western Europeans on coming to the National Computer Conference."

Sophisticated electronics, computers and peripheral equipment such as softwear are products in which the United States is seen as having a competitive advantage over other countries. The United States, which ran a $5.7 billion trade surplus in that field last year, controls between 60 percent and 70 percent of the world computer market. The Commerce Department expects this year's trade surplus in the computer field to grow to an estimated $6.7 billion with both imports and exports increasing.