Multi-currencies is a feature of the phpBB Cash Mod. The phpBB Stores Mod pulls its currencies from that mod. Currencies can be exchangeable.
Personally, I like that setup because you can then define currencies for more purposes than buying items.
I'm no coder, but based on my knowledge of using the phpBB Cash Mod, I'd say implementing its features, especially multi-currencies, would involve heavy coding. However, I do think it would be a very useful addition.