First ---Thanks for bring up some important points to this problem all worldwide collectors are finding more difficult to address and it is also changing from methods of the past . Years ago stamp dealers build huge inventories for their customers and they {the dealers} took WANT LIST from customers .
The world of stamp collecting has change, but I have not seen anything in the philatelic press or on the chat boards where this subject has come up and gave it little thought .
So where do you find the stamps that you need to fill those spots on your 90% completed album page . This problem comes in two ways .
First --The stamp you need but it catalogs at less than $2.00 , No seller is going to sell it cheap by the time the seller figure
ebay fees and postage plus the time to list it and time to mail it out . But the buyer is trying for a low percentage of catalog . With all that why end up selling it ,just throw it into a mixture box .Nobody is happy .
Second --- For a worldwide collector and even a country collector the most expensive stamps to buy right now are the middle price range stamps percentage wise to catalog price . The cheap stamps come to you between 2 cent to 10 cent each and no problem finding them .Buying mixtures , albums and even red boxes full of stamps with catalogs values up to $10.00 catalog can be found at most large auctions and your paying 2 to 10 cents each in large lots .
Right now you can buy $80.00 to $1,000 stamps at auctions from 1/10 catalog up to 1/3 catalog and you will run out of money well before those firms run out of stamps .
Now we get to those stamps that your going to pay the most for base on catalog value . Those are the stamps between $15.00 to $80.00 , it is like every dealers wants a high percentage of catalog or even full catalog for that missing stamp ,this holds thru on
ebay too !