Mon makes the most sense to me to be the currency of Gensokyo. Even though Yen would have existed for a few years before the barrier would go up, there is nothing backing up the worth of fiat currency afterwards. I believe the inhabitants would be more familiar with the Mon due to its longer circulation period.
Also, there's an Ability Card in UM that clearly depicts Mon in it:
There's also the golden tiles in UM that represent a currency for buying and trading Ability Cards. It's likely just meant to be an in-game sort of thing only, but I like to think it could be used in a fangame or something since it now exists as a type of currency in canon.
Muenzuka is pretty much where outside world things turn up. From what I've read, objects from the outside world, especially tech, is kind of worthless. Rinnosuke found and identified Smartphones, and sold them at his shop as fancy paperweights in AFiEU.
I don't really think there can be a smuggler ring given the microscopic list of characters who can freely pass through the barrier, and even less who'd actually care about that sort of thing. The only character I can think of who would might be interested in that is someone like Mamizou.