This question came from a customer and involves used stamps cataloging $50 or less which are generally affordable by most collectors. There are several cases where used copies are hard to find. Here's why.
For example, take Scott #756-765, the Farley imperfs. When they came out, collectors gobbled them up as an investment. Because they didnít have gum, they were hard to use as postage unless you supplied the glue. The price of a used copy is less than the price of a mint copy. No one is going to lose money by having their mint copies cancelled. The few used copies available are generally favor cancels that never went through the mail. A genuinely used copy of Scott #756-765 on cover is scarce. But because collectors value the mint stamps more, I doubt that a genuine used copy on cover would sell for very much.
Many of the imperforate regular issues were created so that private vending machine companies could apply their own perforations for their own vending machines. Private perforations are recognized by the Scott catalog, but they are towards the back of the catalog. Some collectors shun them because it isnít a perforation that the BEP applied. Many of the private perforations tend to be large and/or poorly centered so that they cut into the stamp design which makes them even less desirable. The general public didnít have access to the imperforate stamps since they were mainly created for the private vending machine companies. And if they did, the general public wouldnít use them much. Why spend my time using scissors to cut up stamps when I can buy the ones with holes that separate much more easily? So you donít find many imperforate stamps used unless they have private perforations applied.
There are a few cases where a used stamp is worth more, like Scott #39. Faked cancels exist on these stamps. A genuine used copy is about $10,000 catalog value and a mint copy is much less. There are more mint copies available than genuine used copies.
But for the ďcheapĒ stuff, collectors will probably always tend to favor having a mint copy that is untouched by any postmark. No matter how scarce a used copy is, the mint copy is just as affordable. For many years now, new collectors have been taught by others to only collect MNH stamps because they are worth more. Which is wrong in my opinion, but thatís for a different topic.
Demand is king. Because mint copies are modestly priced too and there is more demand for them, they have the higher catalog value. Demand for used copies is less and no matter how scarce they are, the value will be less. That leaves pesky holes in our collections and we scratch our heads wondering why we can't find that cheap stamp.