From DDO wiki
 Auction House
The Auction House is an automated way of buying and selling items with other players. Players can put items up for sale for up to three days. At the end of the auction, the item sells to the player who offered the highest gold amount for it, or returns to the seller if no one bid. Auction House prices and fees are unaffected by your Haggle skill or vendor reputation.
Items and gold gained from Auction House activity are sent to your character's mailbox. They are not delivered through the Auction House panel.
The Auction House interface is rather intuitive. You can filter items by type, minimum level, maximum level, if the item has a buyout, and (since update 7) any string(s) - words - in the item's name. Clicking column headers in the Auction House display sorts the results by that column, and (since update 7) affects the entire set of results for your search.
Auction house Non Player Characters are marked on the zone map as red gavels.
 Buying Items
VIP and Premium accounts can bid on up to 50 auctions. Free-to-play accounts can bid on up to 20.
Making a new bid costs at least 1.05 × the current high bid. (Clicking an auction item shows the exact cost to bid on it.) This makes it impossible to win an auction by offering a mere couple of copper pieces more than someone else. The only way to buy an item immediately is to pay the buyout price, if there is one set up.
You cannot cancel a purchase bid while you are high bidder. If someone outbids you, your bid is refunded in full and immediately mailed back to you — you do not have to cancel your bid manually to get your money back. In fact, manually canceling an outbid notice does nothing except remove that notice from the panel. An outbid notice disappears on its own when the auction time ends or the item is bought out.
The maximum amount you can bid is 2.1 million pp, but because you must bid 1.05 * the current high bid: the maximum amount you need to bid is 2.0 million and 1 pp.
 Selling Items
Free-to-play accounts can post one auction at a time. Premium and VIP accounts can post up to 50 auctions at a time. You will get a "You cannot post any more auctions." error when you reach the 50-auctions limit. Reaching the limit prevents you from bidding on any further item, and prevents you from using the "buyout" option as well. It is however still possible to raise bids or buyout existing auctions. The restriction is lifted only after some auctions are removed one way of another (sold, expired, or canceled).
You can set the starting bid of an item to any amount of money, though not less than 10 pp. You don't have to set a buyout price, though if you do it must be greater than the starting bid and less than 2,000,000 pp.
If you put an item up for sale, you can only cancel your sale offer if it's less than an hour old or if no one has bid on your item.
It costs gold to put an item up for auction. This posting fee must be paid up front and is refunded if the item sells. If the auction is canceled or doesn't sell, the posting fee is not refunded. The exact cost depends on the item's base worth and the auction's duration.
See also: Shard Exchange