Having both currencies from the beginning makes it easy to understand when you find a place that uses one of them, but as the game is focused on exploring I'll go with the first idea and diplaying some kind of message about the dicovery of a new currency to make it easy to understand by the player.
The trading could be improved by lowering a little bit the prices of the more expensive objects as when you start thinkning about the amount of trades that you need to make in order to buy one of those it feels quite hard when there are some objects that might end up giving you like 2 or 3 coins of benefit per unit. But the system actually works, is just a little bit of polish on that side ^^
Regards :)