The fake, postally used Machins that I have seen advertised on eBay
seem to sell much better than genuine modern stamps. I bookmark them occasionally, and they all sell at prices above the minimum bid. This indicates to me that fake stamps are much less of a problem than skips/reused stamps.
Many of the auction lots I receive from the U.K. are franked with reused stamps. I know this because I remove the undamaged stamps from the envelopes. It is obvious when the stamps are reused; they are much easier to remove.. Also, some will have bent and torn corners.
The idea of postal workers closely scrutinizing mail seems a little ridiculous at the present time.
By the way, here are the 2 pieces of mail I received today with postage stamps. Notice that the U.S. piece has the canceller programmed/shifted to the left, where it will not cancel stamps put in the correct place. There was an earlier thread on SCF discussing this practice.