Looks like there's a pretty exact answer here:

Joan Robinson and B. L. Hallward


Retrospectives: How Joan Robinson and B. L. Hallward Named Monopsony
     Robert J. Thornton
     The Journal of Economic Perspectives, Vol. 18, No. 2. (Spring, 
2004), pp. 257-261.

JSTOR Stable URL:
http://links.jstor.org/sici?sici=0895-3309%28200421%2918%3A2%3C257%3ARHJRAB%3E2.0.CO%3B2-G 



Also, see

http://www-personal.umich.edu/~jdinardo/def.html which says:

"From the introduction to Monopsony in Motion by Alan Manning

The Oxford English Dictionary credits the word monopsony to Joan 
Robinson (1933) though she credits it to B.L. Hallward a classical 
scholar at Cambridge who though born in 1901 is still alive at the 
time of writing. The suffix is derived from OPSONEN which means to 
make your purchases often of dried fish and which is found in 
Aristophanes, the Wasps (twice), Plutarch and the New Testament. The 
natural ONEOMAI (buy) was rejected as it does not sound good with the 
MONO prefix (personal communication to David Card). The invention of 
the word oligopsony is credited to Walker (1943) who introduced it 
with the curious phrase ``it is surely only a matter of time before 
market situation number 23 is christened oligopsony", the time 
referred to being the time necessary for him to finish writing the sentence. "


OK, I guess I can't stall anymore and must get back to grading. Sigh.


Humberto Barreto