I imagine that in many areas of ME a true monetary economy did not arise, and in others (eg. most of Eriador) it disappeared on the occasion of a breakdown in society and political life.

It is probable that in the areas where the common language of Westron was known some form of standardised exchange rate was found based on a common system of weights - "The Numenorian Talent" if you like. So under normal conditions a quarter talent of silver would buy two chickens or five loaves whether you were in Pelargir or Hobbiton. Of course famine or local gluts would tend to introduce variations of this exchange rate over both distance and time. In this system a coinage would not be strictly necessary; though the merchant would have to carry a balance to weigh precious metal and have some rough method of checking purity. Plus for many transactions a straight barter would be possible.