Plenty of places in the world accept currencies besides the local ones because they are more stable or just worth more in general. Depending on the business, they might handle currency the same way too.

I usually see this only in tourist traps, where the EUR/USD price is 120% or more of the "local" price.

Oh boy. Go to Ko Chang or even Vietnam with plenty of ironed-out bills and see how much the locals like you more.

When I sold on eBay and they allowed cash/bank draft payments, I accepted local currency, but also major currencies like US$ (in cash or bank draft because I had a US$ account too) and other major currencies in cash (like EUR and GBP) because they were cheaper to convert with when travelling (or I could use directly). Without a spread.

Used the EUR in Cuba when a buyer mailed me cash.

> How did you convert it for free? Not even actual forex businesses can do that...

Actual forex businesses convert currency at negative cost. That's what it means to be a forex business.