The idea of different forms of money has always inrigued me as way to add some more difference to Ravenloft.  I mean lets get real, in the medival times, money was power.  Money hired mercs, bribed church and state officials, provided emergency food imports to stave off peasant riots, and could hire assassins to take out those who failed to pay tribute.
  Each realm has it's own coinage, from the Sithican Iron Mark to the Barovian Gold Eagle to the Har-Akir barter system.  Another way of making the PC's feel isolated is the fact that many countries will only take the money of other lands at a reduced value, if they will even take it at all.  There is nothing worse than a party who has traded all of their gold for Iron Marks, and turn around to discover that it is worth next to nothing in Falkonovia.

  Barter is a major section of trade that has been forgotten, along with the skill of bartering.  A skilled barter could get an item for less than the value if he out bartered the salesman, and those who have mastered bartering are known through marketplaces far and wide.