Ah, well, as I suspected, there is no convenient definition.  But of   
course, if we are serious about this, there is no such thing as a   
"pure free market"--even theoretically.  Because any exchange   
presupposes a prior agreement on participation in a civil society   
and, more specifically, what constitutes property (you can't exchange   
what you don't own).  The favorite "Robinson Crusoe" (or is it "Jack   
and the Bean Stock"?) example of "free" exchange presumes Crusoe (or   
Friday) doesn't just whack the other guy on the head.  I.e., that   
both Crusoe and Friday bring with them a cultural framework that   
recognizes the value of collaboration.  The absence of civil society   
puts us in the "state of nature"--and whether Hobbes or Locke, it   
always reduces to a state of war, not a market!  
  
Fred Carstensen