From RetroMMO Wiki

Gold is earned from defeating monsters and other players in battle or by selling items. Gold can be used to buy new items in the Shop, Clothier or Armory. Additionally, gold can act as a component in trading or used to restore a party's HP and MP at the Inn. Players may store their gold in the Bank.

The maximum amount of gold a player can store in the bank is 999,999 and 999,999 for each character. For example, a non-member player can have 2,999,997 gold in total.