Auction House

Deposits

Deposits are based on the Merchant Sell Value (MSV) for the item, and the duration selected for the auction, as follows:

In factioned houses (Alliance and Horde owned houses):

Deposit (12hr auction) = 0.15 * MSV 

In the neutral houses (Goblin owned houses):

Deposit (12hr auction) = 0.75 * MSV

For the longer auctions, the base deposit value is scaled by the new length in time. Note that the base deposit is rounded first, and then multiplied as follows:

Deposit (24hr auction) = 2 * Deposit (12hr auction)
Deposit (48hr auction) = 4 * Deposit (12hr auction)

So, ignoring rounding differences we have this rule of thumb:

In factioned houses:

Deposit (12hr) = 15% of MSV
Deposit (24hr) = 30% of MSV
Deposit (48hr) = 60% of MSV

and in the neutral house:

Deposit (12hr) =  75% of MSV
Deposit (24hr) = 150% of MSV
Deposit (48hr) = 300% of MSV

Note that if an item has no sell value to a merchant (MSV=0), then there will be no deposit charged to list the item on the auction house. Low-level bullets and enchanting reagents are common examples of items with no sell value.

The deposit is paid immediately when you list the item.

Suggested Bids

The bid suggested when you place an item up for auction is:

Suggested Bid = 1.5 * MSV

The minimum starting bid is 1 copper. Items with no sell value (MSV=0) will suggest a bid of 1 silver.

Fees

When you successfully sell an item on the auction house, the house will take a percentage of the winning bid as its cut. This percentage is:

In factioned houses:

Cut = 0.05 * Winning Bid

In the neutral house:

Cut = 0.15 * Winning Bid

The amount of money you will receive by mail will be the winning bid, minus the cut, plus the return of your deposit:

Amount Received = Winning Bid - Cut + Deposit
    (in Faction AH) = Winning Bid*(0.95) + Deposit
    (in Neutral AH) = Winning Bid*(0.85) + Deposit

As of Patch 1.9, you will receive a statement along with your payment that shows the winning bid, house cut, and your returned deposit.

When an item fails to sell, you will receive the item in the mail, but your deposit will not be refunded.

Canceling Auctions

When you cancel an auction that has no bids placed on it, you simply lose your deposit, and the item is returned to you by mail.

Cancellation Fee = Deposit

When you cancel an auction that has a bid in place, you will lose your deposit. You will also be charged a fee equal to the house cut of the current bid.

Cancellation Fee = Deposit + Cut

Tips

• Don't lower your starting bid below 70% of the suggested amount. (70% in a factioned auction house, 79% in the neutral house.) Otherwise you risk getting less money for an item than a normal merchant would give you.

Starting bid     = 1.5    * MSV, 
70% of that      = 1.05   * MSV,
Minus the 5% cut = 0.9975 * MSV

• Sometimes you can unstack items to get free auctions. For example, say you are selling fish with an MSV of 4cp. The base deposit on 1 fish is 0. (0.05 * 4cp < 1cp) But the base deposit on 20 fish is 4cp. (0.05 * 80cp = 4cp). Since longer deposits are multiples of the already rounded base deposit, you can list single fish for any length of time with no deposit. This is particularly handy when your fish is worth considerably more than its MSV to other players.

• It may be advantageous to buy out your own auction (using an Alt on another account) rather than canceling the auction and paying the penalties. For example:

Selling an item with MSV = 1g, starting bid of 2g, buyout of 5g.
Set as 48-hour auction, so deposit = 60% of MSV = 60s.
Someone bids 2g, but it doesn't look like you'll get the 5g you want.
If you cancel the auction, you lose the deposit (60s) and the cut (5% of 2g). Loss = 70s.
If you buyout the auction with an Alt, you only lose the buyout cut (5% of 5g). Loss = 25s.