The currency of the civilized world is Gold Coins. The player starts off with a small amount, but this won't last long.
Money can be aquired through hard work, rewards, selling valuables, or finding it. The amount the player has is displayed in the user interface, and in the inventory.
It can be hard to come by, but there are repeating tasks which offer an endless supply, given enough hard work.
Some NPCs allow you to barter with them. The shop interface allows the player to use their shop. Typically this is shown when player agrees in a dialog. The bartering power of the player can be increased by holding an Enhancement Item in their inventory.
To spend money is done through a trading user interface (which appears in certain conversations with NPCs), or handing it over to an NPC in payment for a single item or service. Typical things which can be purchased are provisions, weapons and equipment or a service such as cartography or smithing. The player can also purchase information from the right person.
To buy an item (providing you have enough Gold Coins), simply click on it from the list of items to purchase.
To sell an item (typically plunder), simply click on it from the list of items to sell. You cannot buy back these items once sold.
You cannot sell keys or Quest items (as they are used elsewhere).