Most people who collect R.P.O. covers collect them as part of a geographical postal history collection. They are fit in with D.P.O.'s, machine cancels, advertising covers and such. Therefore, the market is strictly geographical. With some exceptions, there is a fairly even distribution of demand. Michigan R.P.O.'s are in the most demand, followed by R.P.O.'s of California and Florida.
Demand is weakest for the south central part of the country and the southwest.
Streetcars have had some fairly strong adherents over the years. There are a number of very rare items that a potential collector would need to acquire, so new collectors have been scarce. The existing cadre of collectors now only seeks the very rare marking or unusual usages of the more common markings. This market is on a definite downward spiral. While overall R.P.O.'s sell well, the lack of collectors collecting R.P.O. covers for the sake of R.P.O. covers, there is a real possibility that when the current generation of researchers go to their reward, there will be no one to continue their work.