A hundred years ago, Richard Sears and Julius Rosenwald combined to form Sears, Roebuck & Co. About the same time, Aaron Montgomery Ward started his company. The three men thus became the nation's first mail-order millionaires.

Today, as most Americans know from the number of mail-order catalogs and brochures dumped off at their doorsteps, the mail-order business is booming. The catalogs and brochures may be considered "junk mail" by many, but to a growing number of small business men and women the catalogs and brochures represent a good living.Paul Mutchner, chairman of the National Mail Order Association's Advisory Board, estimates that there are 10,000 consumer-oriented mail-order catalogs distributed today and that another 10,000 are aimed at business customers. He thinks the industry is growing between 15 percent and 20 percent a year and that mail-order sales generate at least $50 billion in sales annually.

It almost goes without saying that just about everything is being peddled by mail - from clothing, candy and cemetery lots to jewelry, burial and life insurance, lobsters, kneeling pads, bird feeders, flower and vegetable seeds, folding bookcases and movie cameras.

"Regardless of price, regardless of size, anything that can be sold can be sold by mail," says Louis E. Rudin, head of his own mail-order firm in Highland Park, Ill. "There is no limit to what can be sold by mail, except for the limit of the seller's creativity and the seller's knowledge of consumers and markets."

While some large companies are well-known direct mail marketers (Sears, L.L. Bean and Burpee, as examples), most industry firms are small. A brochure distributed by the Direct Mail/Marketing Association says that one can start a mail-order business with a small investment (the usual figure cited is $10,000) as a part-time business by a housewife who has a sense of business involvement and a desire to earn extra money, as an added profit dimension for a retail store and as a long-term project for the active businessperson who is looking forward to retirement when he or she will need additional income.

William A. Cohen, chairman of the marketing department at California State University (Los Angeles), has written that "your chances for building a profitable mail-order business are increased if you possess the following essential qualities: imagination, persistence, honesty and knowledge."

Cohen goes on to say that "imagination is needed to visualize the special appeal that will compel a potential customer to buy your product. . . . Persistence is required because success is rarely instantaneous. . . . Absolute honesty is necessary because a successful mail-order business is built on trust, satisfied customers and repeat sales."

Not all mail-order firms apparently have listened to Cohen's last point about honesty. The level of mail-order complaints is matched only by complaints about auto repairs, according to Raymond L. Rhine, mail-order program coordinator for the Federal Trade Commission. FTC enforces the one federal law to help mail-order customers. Under the statute, companies must ship goods within 30 days or within the time period stated in an advertisement. If shipments are late, customers are supposed to be offered a refund.

Edward M. Mayer Jr., past educational director for the Direct Mail/Marketing Association, says individuals interested in starting a mail-order operation should make sure their mailing lists are up to date and spend time jazzing up the advertising copy. David Ogilvy, in his book "Confessions of an Advertising Man," says the most important of these buzz words are "free" and "new." Other catchy words are "amazing," "bargain," "just arrived," "miracle," "remarkable," "sensational" and "at last."

The power of mail-order advertising words has been illustrated in a story about an author named Maxwell Sackheim. He sold 500,000 copies of a book simply by changing the title (equivalent to a headline in a mail-order ad) from "Five Acres" to "Five Acres and Independence."

Hard as it might be to believe - given the present proliferation of catalogs and brochures - new and novel products still are coming out of mail-order houses. Not too long ago, for example, Richard Thalheimer began selling a chronograph watch - offering it for sale through a small ad in the magazine "Runner's World." Today, Thalheimer's multi-million dollar firm, Sharper Image, offers a wide variety of unusual sports and recreation-oriented products through the mail and at retail outlets around the country.