You don't. Trade is perfectly legal, if the seller wants what you have, and can make up the difference in trade values. You only need to pay the government in your country's currency. You can even use gold and silver as storable mediums of exchange. You won't get a native trade rate, but that's not something you're going to see in a non centralized currency market. Sellers can't trust a non-cenralized currency to hold it's value from sale to exchange and holding a variety of currencies is a pain in the ass.

I'd love to know I'm what ways it is a scam though. We've been through all these challenges and yet some folks read an article online and are suddenly fiat currency experts.