WHEN NEWSPAPER and magazine readers across the country last summer responded to advertisements purporting to sell "genuine emeralds" for $5, many thought they were buying them from a man named H. M. Fisk, a gem dealer "known in the trade," who was clearing his warehouse.

Few were aware that H. M. Fisk existed in name only; that some of the businessmen involved in the sale had, only months before, signed a consent agreement with the Illinois attorney general's office offering refunds to respondents in a similar diamond-selling scheme under a different company name; that some officers of H. M. Fisk were also officers of several other companies named in a cease-and-desist order issued by the Federal Trade Commission.

Those who plan to do their Christman shopping through the mails will be glad to know that H. M. Fisk is not representative of the $30 billion mail-order business. Some of the largest mail houses have established reputations for competent and reliable service. But enough dissatisfied customers have reported bad experiences that shoppers should be cautious.

In the last three years, the Federal Trade Commission has received more than 20,000 complaints regarding mail-order firms.

Chief among these, said an attorney in the FTC's compliance division, is receiving merchandise late or not at all.

Shoppers should read catalogues or ads carefully for shipping information. There are specific laws governing mail-order practices. If, for example, the company promises to ship within six weeks, then you should be able to expect the goods to arrive within six weeks. If no delivery time is given, the company must, under the law, ship within 30 days.

Sometimes the mail-order house will send notice that it is having problems filling the order and that you should expect a delay. Delay notices must be accompanied with a prepaid means of reply. You need not accept any delays. If you choose to cancel the order rather than wait, the company is required to refund your money, in whatever from you sent it, within seven business days.

If the expected delay is less than 30 days, then you must respond in order to receive a refund. Delays of more than 30 days, however, require your explicit consent. If it is not given, the company must automatically return your money.

Often, the FTCspokesmen said, consumers complain of unfulfilled promises but have no record, and little recollection, of what they were led to expect. "We receive a lot of calls from people who don't even know the name of the company they ordered from," he said.

When ordering by mail, keep a copy of the order blank. If you pay by check, keep a record also of the check number. An wait at least as long as the promised date before calling the FTC.

Also look for the conditions under which the company will accept returned mercnahdise for refunds. Under FTC regulations, the company must deliver a refund as long as the goods are returned within the given time limit, or, if no limit is given, within 30 days. If merchandise is damaged, write the company first, describing the problem, and wait for a reply before sending it back. The product is the only evidence of your complaint.

Some business practices are downright deceptive. But that doesn't mean they've disappeared. Advertisements written carefully to avoid lawsuits are well known around newspapers and magazines. They deliver exactly what they promise, but not necessarily what the customers were expecting.

Read ads all the way through the fine print and don't rely on pictures, which sometimes bear little resemblance to the actual product. And don't expect more than what you're paying for. Deals that appear too good to be true usually are. Add up all the listed costs, including shipping, handling, taxes, etc., to be certain the price of the item is actually less than you might pay at the local shopping mall.

If you do encounter problems, you can call the local consumer-affairs office or better business bureau, or any of the following:

The U.S. Federal Trade Commission, Correspondence Branch, Sixth and Pennsylvania Ave. NW, Washington, D.C. 20580.

The U.S. Postal Service Office of Consumer Affairs, 475 L'Enfant Plaza, Room 5806, Washington, D.C. 20260.

The Direct Mail Marketing Association, 6 East 43rd St., New York, N.Y. 10017.