The amount of copper loaded onto a shopkeeper is the maximum amount they will ever have on their person, and represents the maximum amount they are able to spend to buy an item.

Money they make above and beyond this amount is deposited into their bank account. They will periodically withdraw money from their bank account if their cash on hand drops too low.

If the Galadon wizard doesn't have enough cash on hand to buy something, it's likey he just got cleaned out before you showed up, and hasn't had time to run to the bank yet.

Kastellyn the Devourer of Magic, Lord of Legends