Thanks to Auction Broker available from quick menu you can access Auction House from anywhere. Auction House is a place where players sell or buy various items, only currency of AH are Astral Diamonds.
AH window have 3 tabs: Browse, My Consignments, My Bids.
Using Browse tab you can search for any item you want, after choosing category and setting up item filters you'll see list of results. On the bottom of the window you can use Place Bid button to take part in this auction or (if there is such option) Buyout item immediately.
If you want to sell an item you can do that in My Consignments tab, choose item for sale, set starting bid, buyout price if you want and duration. In My Bids tab you will find items you are bidding on.
Diamonds and won items will be send to you via in-game mail.