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.