Dear HES,
I am sorry for being a little imprecise in my first query.  The term 
monopsony (meaning a demand side monopoly) was attributed to
the discussion on monopolistic competition during the 1930s (as 
Humberto has shown). But I was looking for earlier quotations. 

I know that some scholars of the school of Salamanca (e.g. Molina) were 
concerned with the phenomenon, but did they already use the term? And 
what about the mercantilists, who where concerned with "market 
structure" as well (if you think of J.J.Becher). Did some of them use 
the term?

Thank you for any kind of help!

Jan-Otmar Hesse