Fun fact: Charles Ponzi (yes, THAT Ponzi) got started buying and selling postage stamps via arbitrage. It was legal until he started doing some shenanigans regarding over-promised ROIs and needed to use new investor money to pay older investors.
He would buy a stamp in Spain for 30 centavos, then exchange it for a 5 cent stamp in the US. Spain’s currency was depreciating against the dollar, so you could make some surprising returns. Really, his numbers weren’t straight-up lies, it’s just that it would’ve been impractical at the time to transport enough stamps to make that payout occur.
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.
He would buy a stamp in Spain for 30 centavos, then exchange it for a 5 cent stamp in the US. Spain’s currency was depreciating against the dollar, so you could make some surprising returns. Really, his numbers weren’t straight-up lies, it’s just that it would’ve been impractical at the time to transport enough stamps to make that payout occur.