About three years ago "Moneywise" noted that John Sehlmeier (Box 5324, Oxnard , Calif. 93031) offered to pay 25 cents for worthless stock certificates to enable owners to take a tax loss. Establishing the worthlessness of stock holdings can be difficult, but a legitimate sale of the stock, even for 25 cents , clearly establishes the final value. Stockbrokers will seldom process a sale to establish a loss or charge a minimum fee that could run $25 to $40. Of course, an owner must collect whatever facts may be necessary to establish in his own mind that the stock really is worthless. He should also have the documentation that establishes the price originally paid. Any long-term loss thus established may be used to offset long-term capital gains dollar for dollar or ordinary income on a 2-for-1 basis up to $3,000 each year, with excess carried over for as many years as necessary to use up the loss. Anyone wishing to sell a worthless-stock certificate should sign the certificate on the back exactly as written on the front and send it to Mr. Sehlmeier. He will send 25 cents. Any 1981 losses will be reported on 1982 tax returns. But it should be noted that some old stock certifi cates are valued by collectors at far more than 25 cents.