Also, I'm curious what the real money (not bullet money) will be used for.
There's a Card Shop on the market selection screen, and you even get to see the number of your gold items there (1st screenshot from the official 2hu news site), so that's probably where you use your normal money. It seems like you'll have a selection of 5 cards to buy (going by the last screenshot from the official 2hu news site), could be more, could be less as we go on.
Basically, here's how it'll go :
-Bullet money is created by using Marisa's magic attack thingy on enemy bullets
-You use the bullet money you have to buy cards at the black market
-Money (gold items) randomly drop from killed enemies and can also be generated by other means such as the miracle mallet
-You use the real money to buy cards at the item shop
Speaking of cards, the Eternity item card featured in the screenshot seems to have 0/1 written on it. Maybe there's some extra condition to fulfill in order to properly unlock cards or something.