Gold, diamonds and the market

The two currencies

Gold — the main currency. It comes from battles, exploration, quests and sales. It pays for the Tavern, refining, crafting and most trading.
Diamonds — premium currency, used for cosmetics and some auction listings.

A fixed-price gold shop: consumables and basic items, always available. It is the game's price floor reference.

The player-to-player market.

You list an item from your bag setting a price and a currency (gold or diamonds).
The item is locked while the listing is active — you cannot equip, refine or relist it. If it was equipped, listing unequips it.
Listings expire after 48 hours.
On a sale the system charges a 5% fee: the buyer pays full price, the seller receives 95% and the difference leaves the economy (that is what keeps inflation in check).
You cannot buy your own listing.

An expired or cancelled listing returns the item — nothing is lost.

Items have a fixed sale price. It is the quick way out for inventory junk; for good items the auction house usually pays more.