In attending a human relations program some time in the past one of the instructors taught us that to assume (ass-u-me) is to make an “ass of you and me.” Unfortunately, there is much too much assuming in the world of philately, where the facts are not known and an assumption is made. 
There are enough examples of this in the stamp world, and clear examples of this abound when one goes through the KPS catalogue. It is too often assumed that a used stamp or SS should be priced at a fraction of the mint value. But if one then goes and tests the market and tries to get a used one of this or that, it is then found out that some used items are really hard to find. If a used stamp is very hard to find, I don’t feel that the catalogue should assign a low value to it. But to give credit where credit is due, the KPS catalogue has hesitated to give a value for used SS in most places, as well as the value of an SS in FDC. Time is needed to work out some of these values or for a person who is brave enough to say how much an item is worth by knowing the market and the supply.
I’m not very specialized in South Korea stamps, but to my understanding, in the old days, KoreaPost issued “new stamp infomation” in small pamphlet or sheet, with a real stamp affixed to show the stamp image. To avoid that stamp being taken down and sold to clients, that stamp will have a chop of 견본, means specimen. ( I prefer using specimen instead of sample, which is already a philatelic term now). So, SK specimen available on philatelic market are always with a affixed trace on back and gum was not complete. But these specimen are mint and should be sold at least as the same price with mint stamps.
Technically this stamp should not be sold to clients for usage, however, sometimes the post office staff didn’t notice the differences but sold it, that caused a rare cover.
I agree that modern fake specimen are easy to produce if you get a rubber chop, however, since modern SK stamps are not expensive, making fake stamp is meaningless.