SOUNDING VERY much like a salesman with a difficult customer, Secretary of Agriculture John R. Block was in Moscow last week buttering up the Russians. The purpose of the trip was to sign the new grain agreement and to pledge that the United States would never, never embargo agricultural sales to them again. In return for a Soviet commitment to buy at least 9 million metric tons of grain a year, the United States promises not to retaliate for Soviet geopolitical misbehavior, like the invasion of Afghanistan, by holding up shipments of the one American product that the Soviets really need. In all matters related to farm products--if not in certain other questions--the Russians are henceforth the valued friends and partners in trade of the Americans. "We want not only to be a good supplier but the best supplier for the Soviet Union," Mr. Block enthusiastically declared.

As Mr. Block's words suggest, the Reagan administration has now arrived at a clear and firm policy on economic relations with the Soviet Union. It opposes letting them buy those things that other countries want to sell them, but it favors letting them have anything that Americans want to sell. As a practical matter, Mr. Block's grain agreement probably will mark the end of any serious attempt by this administration to use economic sanctions in dealing with the Russians. That's just as well. Economic sanctions are not always a bad weapon, and President Carter was justified in using them in the case of Afghanistan. But they are not well adapted to an administration, like Mr. Reagan's, that is not prepared to deal with the outcry from American farmers.

North American and Western European governments are all anxiously competing in their efforts to ease the painful process of agricultural adjustment through export promotion. There were grain shortages a decade ago to which governments responded by encouraging enormous production increases. That has created the present very expensive surpluses. What to do? Try to get the farmers to cut back a little and, in the meantime, see if you can't get rid of the stuff abroad.

The purpose of the Soviet agreement is partly to try to stabilize Soviet purchases, which tend to swing up and down outrageously. Partly, of course, the purpose is to show American farmers that their Agriculture Department is out there battling for them, and trying to preempt for American exports the largest of all foreign markets. In reality, except in the case of a real world shortage, agreements like this one have little practical effect on exports and prices because they don't have much effect on worldwide supply and demand. But farmers find these agreements reassuring, and successive American administrations have learned to take Russian grain deals very seriously for reasons that do not necessarily have anything to do with economics.