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Most people use stamps to mail letters. Marc Rousso trades them for expensive houses, yachts and cars. A Miami Beach philatelic broker, Rousso, 35, says that official catalogs value the rare stamps that he has traded since 1984 at $45 million. Now he is taking the art of stamp trading into the computer age. Starting this week, collectors worldwide will be able to buy and sell rare stamps via telex through Rousso's Miami-based International Stamp Exchange Corp. In a month, stamp buffs will also be able to trade through personal computers.

Rousso expects some 300,000 U.S. collectors and thousands more abroad to use ISEC, for which they will pay 3% buyer's fees and 6% selling commissions. By making trading convenient, he also expects to attract new collectors. To ensure the authenticity and quality of stamps listed on the exchange, ISEC's staff experts will take possession and verify descriptions before posting sell orders. The price a buyer will pay for a stamp, of course, depends on numerous factors. The famous "Inverted Jenny" from the first U.S. airmail issue in 1918 is especially valued by collectors for one striking error: the airplane is flying upside down.