THE NEW TAX LAW permits companies to buy and sell their tax benefits--a bright idea that is turning out to be a classic botch of tax policy. It is giving little help to the hard-pressed companies that it was supposed to help most, while it enormously benefits those to which it was intended to give the least. The rain is said to fall on both the just and the unjust; this tax law sprays subsidies on both the unprofitable and the highly profitable with a fine impartiality.

The original concept had a certain logic to it. For many years the tax code has offered inducements to businesses to invest. But since the inducements are all in the form of income tax deductions and credits, they are worthless to companies that aren't making profits. The Reagan administration wanted Congress to expand those inducements enormously, but it also wanted to cut in the unprofitable companies. Among other reasons, there was the Chrysler case. Neither the president nor Congress wanted to risk the embarrassment of another bail-out bill for Chrysler. To let Chrysler, and similarly unprofitable companies, sell their unused deductions and credits for cash seemed a clever way to do a little inconspicuous bailing without getting the Treasury directly involved.

The moral of the story is that a president should never try to be clever with the tax laws. It always develops that tax accountants and lawyers are equally clever and put these laws to uses of which their authors never dreamed.

Chrysler has sold its 1981 tax benefits for $26 million, a modest amount in comparison with the company's current capital spending--not to mention its losses. But last summer's tax bill is creating such large new benefits that even highly profitable companies find themselves with excess tax credits and deductions to sell. Further, even companies that are paying taxes will often find advantages in selling benefits. In an elegant essay in the journal Tax Notes, a former Treasury official, Emil M. Sunley, surveys some of the ways in which even profitable companies can employ these sales. The lease that is the vehicle for selling tax benefits offers a company wide discretion in moving its tax liabilities back and forth over a period of years to fit them into highly sophisticated financial strategies.

The tax rate for large corporations remains at 46 percent of their net income, but the rapidly growing market in tax benefits means that most companies will not pay most of that tax. Instead, they will buy tax benefits from each other. At what price? It varies. The market is still in a very fluid state. About the only thing that you can say with any degree of certainty is that corporate income tax revenue to the Treasury seems to be falling rapidly toward a point not far from zero.