There is no currency. Trade is barter between two characters, each offering items and both agreeing to the exchange.
Invitation
A trade begins with an invitation. An unanswered invitation expires. The trade window opens for the recipient only when they accept.
Escrow
An offered item moves from the offerer's inventory into escrow, where it is visible to both parties and owned by neither. When both parties confirm, the two offers are exchanged. If either party cancels, every item returns to its offerer.