As the number of distinct collectible stamp varieties passes a million and moves quickly toward two, the importance of catalog values has changed. There is nowhere near enough collectors with enough interest, time, and money to absorb all the newer stamps that come to market, let alone the enormous quantities of older material that is sold every year. Accordingly, the resale market for stamps has evolved into three distinct markets.
First, is the postage market. Every major country’s stamps are useable for postage. (This varies by country, with some European countries stamps only valid in the Euro period to the case of the United States where every stamp issued 
since 1861 is valid for postage) Very modern stamps have been sold in far greater quantities than collectors can ever absorb and so are mostly sold by dealers as discount postage.
Second, are the stamps that make up the bulk of most collections. These are the stamps that catalog anywhere from 25¢ (the minimum Scott price) to $5 or so. They are stamps that are found in most collections of that particular country, and their catalog price represents a “service charge” or a cost of sales. Such stamps sell in collections at 15% of catalog or so and are very common. (Remember, a hundred thousand random common stamps catalog a minimum of $25,000 and you still have nothing that any collector who has been at the hobby for any length of time needs)
Lastly is what can be called collectors stamps—rarer items that are scarce enough that most collectors need them and expensive enough that dealers can afford to sell them individually. These are the stamps that collectors who wish to see appreciation in the value of their collections should concentrate on. Such stamps sell at price up to and exceeding half of catalog value and are the items that are missing from most collections.