Fake perforations have been a problem since the 1930s if not before. The American Philatelic Society was able to shut down an operation making fake flat plate coils in the 1930s. After Richard Kiusalas carefully measured US perforations and produced the US Specialty gauge in 1965, newer stroke perforating machines for the stamp alteration trade were more accurate. I have examples of fake perforations in the reference collection received from Kiusalas when I met him in the 1960s. Soon after eBay
started, an individual from upstate New York would buy straight edges or proofs on eBay
and offer them a couple of weeks later under a different account with newly added fake perforations. Another seller from coastal Southern California was found to buying quantities of unused no gum stamps and selling only "never hinged" stamps. While it is more difficult to trace eBay
sellers who create their own (DIY) fakes, there is no reason to believe that the practice has stopped.