Some dos & don'ts of the tag-sale business

Since many others have thought they would like to launch a similar tag-sale business, here are some of Mrs. Fitch and Mrs. Kain's most important do's and don'ts.

* Do find a partner. The work is much too demanding to do alone, physically and in every other way.

* Accept your friends' offer of help in the beginning. Sort out which ones work well and put them on the payroll. Mrs. Fitch and Mrs. Kain pay a very modest $25 a day, but have a waiting list of people in the community who want to work for them.

* Before you begin, go to your lawyer and have him draw up a contract which states exactly the extent of your services and what you are going to pay for out of your fee. Copies signed by you and the client should be held by both parties.

* Find reputable appraisers for Oriental rugs, paintings, jewelry, and fine furniture or antiques.

* Keep an advertising campaign going in all your local papers, running both individual sale ads and regular service ads that define what you are offering.

* Write or telephone bankers, lawyers, and heads of women's and church groups in your area to advise them that you are there to help them.

* Go to tag and garage sales and auctions to see what your competition is doing and take note of their pricing.

* Do research on the objects you are selling. Study your antiques. Buy price guides, like ''The Kovels' Complete Antiques Price List,'' which is now in its 14th edition. Subscribe to those special-interest newspapers that chronicle current events concerning antiques and collectibles.

* Before you begin a sale, equip yourself well with such items as tags, stickers, marking pens, masking tape, safety pins, scissors, tape measure, magnifying glass, folding tables and chairs, flashlight, hammer, screw driver, assorted nails, and string.

* Invest in professionally made signs painted on wood. Good signs are very important. ''Our big red, white, and blue signs stand alone, feature our logo of an American eagle and our Fitch-Kain name, and they give directions clearly. Now people in our area quickly recognize our signs,'' Mrs. Fitch says.

* Separate everything in the house into categories - glassware in one spot, silver in another, bric-a-brac in another so people don't have to run all over the house to find what they want. It's a courtesy to shoppers, and it also helps sales.

* Have a cashier who is both accurate and quick. At the end of each day of the sale, sit down with your client (or the banker or lawyer handling an estate) and count the money taken in. Honesty is very important in this business.

* When people ask you to tell them what their things are worth, do not give an estimate; you probably do not know, and you could make a terrible mistake. What you reply is, ''After our appraisers have been in, we can give you a pretty good idea.''

* When you have everything sorted, marked, and ready, do not sell anything to anyone before the sale begins - not your friends, relatives, or anyone. When you advertise things they have to be there.

* Don't try to sell a lot of things in one day. Have a two-day sale. And don't panic and reduce prices too soon.

* Don't accept checks without proper identification.

* Don't stint on help. You can't watch your merchandise, pack dishes, and talk to your customers all at the same time. (Fitch-Kain had 18 sales assistants at their four-day consignment sale.) Be careful not to hire anyone who is not absolutely trustworthy. This is a tempting business, and people could be corrupted by it.

* Don't be too concerned about occasional mispricing. It is bound to happen because you can't know everything.

* Ask clients to leave the house during a sale of their goods. People become too emotional about their possessions, and they tend to overvalue them. Mrs. Fitch and Mrs. Kain have had women break into tears when seeing their sofas carted away. The partners find their own biggest job is to stay unemotional and as professionally detached as possible.