Wert,
I believe this unethical behavior occurs and some unethical individuals can easily manipulate an unused mint stamp to appear used. only an expert can tell a proper ink cancel.
I'd go a step further and say that one could also do the same, and put it on a cover to make it worth even more. I see a current cover now on
ebay that I am of the opinion is false--to be clear : a real authentic stamp from Newfoundland, changed from mint to used, on an authentic period envelope, with its original stamp removed, and the manipulated authentic stamp affixed in its spot (using the authentic original gum to affix it) and a forged cancel. its a perfect non-forgery forgery. we all need to be very careful to only buy from legit sellers, and one who accept returns without question.