Select the item you wish to bid on in the Browse window. The Quality/Name of the item will be listed with Level (Lvl), Closes In (when the auction ends), High Bidder (current highest bidder), and Current Bid.
Buyout Price
If the item has a Buyout price it will be listed as Buying + a price. This is not the current bid, but instead how much you must pay to buy the item and end the auction now. If this price seems too high, don't worry. The price won't reach that high unless you want to end the auction now or someone else decides to "Buyout" the item.

Auction Length
Check out how long the auction is running. If it's running an extremely long time, you may wish to avoid bidding on that item. Otherwise you'll have to wait that long to get the item.

Outbid!
If you are outbid on an auction you will receive a yellow text message. You will also receive your bid money back in your mailbox. You will need to bid again if you want that item.

Bid Tab
Select the Bid Tab to see the current status of the items you're bidding on. You will also receive yellow pop up messages in the middle of the screen letting you know if you were outbid or if you have won an auction.

Creating An Auction (Selling)
Place Item in Auction Box
Click on the "Auctions" Tab. In the top left of that menu under "Auction Item" there is a box to place your item. Drag an item you wish to sell from your backpack into this box.
Set Starting Price
The auction house will suggest a starting price. You can adjust this by setting it higher or lower. In some cases the suggested price will be much too low for the true value of the item, or it will be set too high. As you buy and sell more items, you can determine a good starting price. If you don't know, just pick the default suggested price. A low price will attract bidders who think they can get a deal on the item.

Set Auction Duration
It's important to realize that the longer you set the Auction Duration, the more the deposit will be. If you set an extremely long duration, the deposit price can be way more than the actual value of the item. If your item sells, you will receive your deposit back. If no one buys the item, you'll lose your deposit. Generally it's good to pick a very short auction time to reduce the risk.

Buyout Price (Optional)
If you want, you can set a buyout price. This is the price at which you will instantly sell an item to a person. If you're selling a bag and think that 85 silver is a good price for it, you can set the buyout price at 85 silver. However, be warned that if you set the buyout price too high, it might discourage people from bidding because they think your buyout price is way too high. It's up to you if you want to use this option or not. Some people really appreciate being able to instantly purchase an item.

Auction Successful
If someone buys the item you are selling you'll receive a text message popup when the auction ends. Go to your mailbox and you'll find a mail from the auction house which contains the money. It will be lower than the sale price because of the auction house fee was taken out of the winnings.

Auction Expired
If your item doesn't sell by the time the auction duration is up, your auction will expire and you will lose your deposit money. The item you put up for auction will be returned to you from the auction house through the mail. You have up to 29 days to get it from your mailbox. Try putting it up for auction again at a reduced price or at another time. If you think it won't be sold, sell it to a vendor instead or try the trade chat channel.

Auction House Tips
Auction Length
The longer you set the auction to run, the more you'll have to pay for the deposit. You should try to set your auction for as short of a time as possible. Otherwise the deposit can be VERY expensive. You could also lose the deposit if the item doesn't sell.