https://corrieredelmezzogiorno.corriere.it/napoli/notizie/ar...
It generally involved coating them with a common substance so that your recipient could remove the ink easily with others discussing solvent concoctions telling you how to remove ink on stamps you received.
Also I recall various posts telling you different ways you could defraud vending machines that had different levels of anti-theft devices. Some involved using washers a comparable weight and dimensions of coins that were cheaper than the coins, some involved building tape structures on paper currency and feeding it into the machine and knowing the precise time to begin pulling it back out with just the right amount of force to get the machine to accept the bill but still retrieve it.
Then of course all of the different phreaking boxes, some of which worked and some of which were pure fantasy.
While I appreciate this sort of thing at a hacking level "I wonder if I can do this", it always amazes me when people that would never steal from a store or a person are quick to try activities like stamp fraud, unlicensed violation of radio spectrum, system penetration/data theft, ripping off vending machines and payphones etc.
Mind you in my youth in the 1990's I tried lots of these things too.
Covering stamps in clear glue so that your correspondent could remove the cancelling stamp when steaming them off the envelope for eventual re-use was something that was commonly discussed during the heyday of the zine scene in the 1990s. Now postage has become so obscenely expensive again for anything beyond a letter that maybe I should get back into the glueing trade... :P
https://en.wikipedia.org/wiki/Charles_Ponzi#Origin_of_the_te...