Actually, harder than making an "Old Put" stamp and putting it on back of a caramel card. The Exhibit sample writing is printed on the card, not stamped after the fact. ESCO later switched to a stamped format for samples:
The difference is readily discerned.
Until recently no one would have even thought to do it because the ROI wasn't there. The price of this sample was a shocker. Plus they are so hard to find that having a good enough example to make a convincing stamp from would have been a challenge. I suppose someone could try it now but I doubt they'd get it right enough to pass muster with anyone who's seen one before.
As for whether the latest one was legit, if that is at issue, the seller is a legitimate dealer who was breaking down a big group of exhibits. My experience in chasing ESCO issues for the better part of two decades is that they tend to come to market in mixed-subject original groups like the seller had. Nothing in any of his cards has led me to worry about counterfeiting or anything else rotten in Denmark about his offerings.
Now, what someone could concoct would be the coupon stamp that appears on rare occasions on back of 1927s:
