Inside the auction, in the selling tab
The Auction House is a place where you can buy or sell items. Anything on the inventory list (which only contains unbound items) can be sold, including equipment, gems, books and other things such as treasure chests, transmutation materials, etc.
On the Auction House you can only buy items with gold, and to put an item for sale you would need to pay a silver fee (If you are VIP you don't need to pay the fee).
The fee is 1000 silver for 6 hours, 2000 silver for 12 hours, or 4000 silver for 24 hours.
You can find this building in the main city of each race: Necrolis, Sanctaria and Arthlan. It is also available in Regia Deorum.