Correction: his idea was that he could buy international reply coupons (which were different than regular stamps) and arbitrage the rates against whatever currency value was greater. As an academic exercise, it was interesting (if utterly unpractical, especially for 1920).

But he NEVER did anything legal. In fact, he didn’t even receive his first attempt at successfully buying a small number of international reply coupons from a friend in Italy until his scam unraveled. And he’d never figured out how he could turn those coupons into cash.

And the reason his arbitrage excuse was caught so quickly (it was basically one summer that he was able to run his scam before his money ran out), was because the global supply of all the international reply coupons in the world were valued at significantly less than his revenues from his schemes, a fact experts pointed out to the local papers as the scam was happening. In fact, he even stopped telling new investors about the postal reply coupons scheme and had moved on to other lies about how people could double their money so quickly.