Many US revenues are quite valuable. Foreign revenues are not very popular in this country and there is very little demand. Dealers and auction houses often price foreign revenues using foreign catalogs. While there is demand in those countries for their revenues, there is little demand in this country. If you get sucked into paying the prices dealers want, you will have a devil of a time getting your money out of them when you sell.
Revenue stamps belong in revenue collections.
When you buy a collection, do not count the revenue stamps unless you collect them.