The doorbell rings. You answer, and there stands . . . a door-to-door computer salesman! Sound far-fetched? Not according to Steven R. Gerbsman and William Binch, president and executive vice-president, respectively, of EKOS Inc. in San Rafael, Calif. They believe that door-to-door home computer marketing is the wave of the future.

"How to profit from the home computer boom . . . !" proclaims the company's brochure. "EKOS has found that people are more receptive to computer sales presentations in the privacy of their homes . . . " To tap that market "you need no prior computer experience, you can keep your present job, and you do not need a great deal of money."

What you need is $99. This sum enrolls you in a five-hour course in the arcana of selling the home computer. And that makes you an independent EKOS sales representative, free not only to sell computers and software at a 10% commission, but also to build a pyramid of second- and third-level sales reps as well. The product line now includes Commodore and Franklin computers, and software from half a dozen companies. But even if you don't sell a thing, joining the EKOS team allows you to deduct the cost of purchasing your own home computer from your taxes as a business expense.

So the computer revolution marches on. The next thing you know, unemployed people will be selling Apples on street corners.