You use the Exchange, which you can open with the little EX button by the mini-map. If you want to use it you need to deposit gold/goldleafs into it. The balance at the top(Transaction Account) is whats in the Exchange, the bottom (inventory) is what you have on hand. Using whats in the Transaction Account you can SELL (goldleafs to gold button), or BUY (gold to goldleaf button). The Exchange will list your offer and when it matches a corresponding offer the amounts are traded. IE: I sell 1GL for 10 Gold, when someone offers to BUY 1 GL for 10 Gold, i get 10 gold into my Transaction Account.
The listed amounts are always PER goldleaf, you must pay a small fee for using the Exchange, and you can withdraw/deposit gold/goldleaf at any time without fee.