How very nice, Humberto! In the last few days of my Intermediate Microeconomics course this semester I developed a handout that employed the terms: monopsony, oligopsony, monopsonistic competition and polypsony. I derived these terms from Fritz Machlup, _The Economics of Sellers Competition: Model Analysis of Sellers' Conduct_ (Baltimore: Johns Hopkins Press, 1952), pp. 126-32, where "Types of Competition in Buying" are discussed. Machlup's sources aren't handy for me at this moment but I suspect he was one of the first, if not the first, to fully develop the parallelism between buyer concentration and seller concentration. Bruce Larson