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