This page is out of date. A newer version of Auctioneer than what is pictured is available. The principle is exactly the same however.
Step 1 Scan the AH with Auctioneer.

On the auction house panel you will see a button called "Scan". Press it and patiently wait for it to scan the current auctions. This can take several minutes depending on how much is up for auction.
What auctioneer is doing is collecting all the pricing and bidding info that the AH currently has. Each time you scan the AH it will remove expired auctions and add the new ones. Over days and weeks this establishes the average values of various items.
Step 2A Use Bidbroker to find under priced items that are about to expire.
Auctioneer has a command called "bidbroker". It will look for auctions in it's last scan that are close to expiring and are under priced by an amount you specify vs market value.
For example:
/auctioneer bidbroker 100
will list all items that are currently priced 100 silver under the high sell point for those items.
Here are the start of the results I got when I ran the "bidbroker" command.

Take a look at that first item. A blue, Skeletal Shoulders. In the chat window you can see what bidbroker thinks of the item. HSP 10g.45s, minBid 5g.10s, Prof 5g.34s, Time: Short.
So what does this mean? And should we snap up this item?
HSP - High Sell Point - Means the highest price Auctioneer has seen for this item. Here is where you need to be careful. This is NOT what you can sell it for, but only the highest price Auctioneer has seen it for.
minBid - Current Bid - What would you need to bid to be the current high bidder.
Prof - Profit - How much could you potentially make from this item.
Time - How soon will the auction expire. Short = less than 30 mins.
Now before you bid on the item, take a look at how many times this item has been to market. In this case 14 - ok we know this item is generally worth putting on the AH. Take a look at the buyout median, 20g, good. Now look at the range of value for those 14 previous items, 9g.01s - 17g.88s - good. Now look at how many of those 14 had bids, 28% - ok people bid on this item. And finally do you know about this item?
After you have pondered all of that, search for the item and place your bid if you feel like this item is re-sellable. In this case I would buy - the min price seen for this item was 9g.01s and people have bid on it. Remember BID do not BUYOUT.
Since the time for auction expiration is short not many more people will see that item before it goes. With a little luck you will win the item for a low bid.
In this game you have to expect to lose some or even many items being out bid. Do not get greedy and get yourself into a bidding war. There will always be other items to go after.
Step 2B Use Percentless to find low buyout prices.
Use the command:
/auctioneer percentless 70
to find buyout prices set at 70% less than HSP.

In this example, the first item is the "Broken I.W.I.N Button. Auctioneer reports: HSP 90g, BO 3g, Prof 87g Less 97%.
Hmm. In this case we have a strange situation. This item is a novelty item that is only worth what people will pay for it. I suspect that someone has been manipulating the market price with this item. It certainly look like a steal, but look: 12 times to auction and 0% bid?! Also the massive money discrepancy make this item too risky.
If you would have wanted the item, you would have simply searched it on the AH and used the buyout to pick it up instantly.
You can set the percentless to whatever you wish. I generally start with 70% to look for some super bargain. Once I sort through them, I'll try again at 50%.
Step 3 Collect your wins and re-sell.
Now that you have won your items (with luck) pick them up at your mailbox. Assuming you still want to part with them, return to the AH.
With Auctioneer installed, when you attempt to place the item for bid, it will suggest pricing based on it's scan data. You'll need to take more care with this step than in the previous. Auctioneer often will not make the best decisions on the bid / buyout prices it sets. Here is what I do.
First drop the item into the create auction window. Next search for the item in the main AH browse window. Take a look at what similar items are priced like. Can you undercut a little and still make money? Are there 10 items on auction and 9 are priced high and 1 low? Try to undercut the 9 higher priced ones and ignore the low priced one (or buy it!*). Go back and adjust the pricing as you see fit.
Read on for some tips and tricks for buying and selling.