The advertisements have already begun to appear. They tell you that only a LIMITED NUMBER of first-day covers will be officially post-marked to commemorate the inauguration of Ronald Reagan as president.

The implication in the ads is: Don't say we didn't urge you to cash in on this once-in-a-lifetime opportunity to make a big profit. Just admit you weren't smart enough to act.

So much for fiction. The facts are that there is no limit on the number of first-day covers (envelopes) that will be officially postmarked in Washington, Pacific Palisades and seven other cities by the United States Postal Service, and there is no guarantee as to what any of these "collectibles" may be worth at some future date.

The advertising hype is intense, so it may be useful to evaluate the claims that are being made.

Donald M. McDowell, general manager of the stamps division of the United States Postal Service, tells me there are "dozens, perhaps hundreds" of privately owned, for-profit companies engaged in the business of supplying collectors with first-day covers that commemorate philatelic events such as presidential inaugurals and the arrival of new postage stamps.

These films supply specially printed and postmarked envelopes for which they charge from $1.50 to $50 (for gold-leaf fancy work). McDowell says almost all the firms are "legitimate" in the sense that they deliver what they advertise. Only a fool risks antagonizing postal inspectors.

McDowell has no comment on the values offered, nor will he speculate on a collector's chance of getting rich by assembling complete sets of first-day covers. He's wise to stay out of that area. All collections, whether stamps, baseball cards, matchbooks or "limited edition" dinner plates (limited to whatever number the company thinks it can sell), are of dubious value until a buyer's cash is in the seller's hand.

Experienced collectors know that one price prevails for buyers, quite another for sellers. Take it or leave it.

One dealer put it to me this way: "Most people never become serious collectors. They don't spend a lifetime putting together collections that make other collectors envious and eager to bid. Most people collect a few things of modest value. They never bother to sell because there is no great amount of money involved. When they die, they leave a smattering of this and a little of that to their heirs."

If you'd like to have a memento of Ronald Reagan's inaugural, two courses are open to you.

You can patronize one of the firms that make a profit by selling you first-day covers for $2, $5, $10, $50 or some other price.

Or you can have your own self-addressed letter postmarked, just as "officially," for as little as 15 cents. This area's residents can mail their self-addressed, stamped envelopes to The Postmaster, Washington, D.C. Or they can walk into the main post office on Massachusetts Avenue on Inaugural Day and get special cancellation service over-the-counter. Deadline: midnight, Jan. 20.

McDowell is kindly inclined toward the privately owned firms that deal in first-day covers because they are USPS customers. However, he concedes that most of his first-day business is done with individuals who do not patronize intermediaries.

"People are incredibly talented in devising their own first-day envelopes," McDowell told me. "They range from crayon drawings done by children to beautiful pictures and inscriptions drawn by talented adults. And people who are adept at photographic processes can create envelopes that bear appropriate pictures and legends, for example President-elect Reagan's picture, or Reagan and his wife. There's no limit to their ingenuity.And of course we put the same cancellation on every first-day cover, whether it was created by an amateur or a professional."

Congress has decreed that the USPS refrain from competing with private firms that sell first-day covers. USPS is permitted to sell 10-cent postal cards for 10 cents (we pay for the postage but Uncle gives us the card free). And USPS is permitted to sell envelopes with first-class postage imprinted on them for 3 cents more than the cost of the postage alone -- a good bargain in these days of high paper costs. But Congress does not permit USPS to provide first-day covers that compete with private entrepreneurs.

Perhaps that's why it's known as Incongruous Assembled.