The Auction House can be opened using
In the Auction House, you can buy and sell items directly to and from other players.
To list an item on the Auction House, use
/ah sellwhilst holding the item(s).
The Auction House main menu, from /ah
You can view your currently listed items by clicking the bottom-left diamond.
The recommended way of earning money via the Auction House is to find commodities that other players demand, and sell them. For example, Mending books are popular.