Let's imagine this scenario.
You and a friend are buying ice cream.
You pick the flavor of ice cream, the vendor puts it in a cup and hands it to you. You reach into your pocket and realize that you are fifty cents short.
A) turn to your friend and say "hey bro, can you spot me 50 cents?"
B) hand the ice cream back to the vendor, step away from the counter and ask your friend to buy it for you?
If we can't have a shared money pool, please at least let us pass the cash between characters via right click or something when we are in the trade interface. And better yet, since our characters are joined at the hip anyway, let's get away from this "separate purses" thing and pool the team money together.