I don't know of any coin that couldn't be used as a coin for greece. It's not like there are many requirements besides being able to use it as currency and the means to exchange for EUR (which would prob be set up by greece if they decided to go that route).
I don't know of any coin that could be used as a currency of a whole nation. The scalability just isn't there.
That's actually a good point.
If the coins offers asset to asset exchange, then why not? Eg 30 assets of eggs for 17 assets of tomatoes
I don't think in a scenario like this anyone would be using an asset exchange. You go to the market just like you do know and then pay with %insertrandomroin% instead of EUR. Do you honestly believe all of a sudden small markets would start to put their entire investory into an asset exchange ? Not only would it be incredibly cumbersome it would imho also be unneccesary.