Add-Ons
Below is a short list on the add-ons I use. If you have any more you would recommend, feel free to comment.
Mail mods
One important thing you will need if you spend a lot of time on the AH, will be a mail mod of some sort. I use Postal, but any mail mod that opens mails will do. If you List a couple hundred or more auctions every 2 days or so, you will have several sold auctions to open and whatever you don’t sell will need to be opened in an ended auction.
There are some mail mods that also help you remember the last person you mailed to (filling in the name in the new mail to send) which can be helpful when mass mailing a bunch of stuff to an alt. However be aware that you can send an item to the wrong person (if you forget to the change the name in the mail box).
Auctioneer
This is the most popular auction house mod. For information on how to use this mod I recommend going to their site and reading their how to guide.
Before you start relying on the data auctioneer provides, I recommend getting at least a weeks worth of data. Prices vary depending on the day of the week so making sure you scan each day helps give you a general idea of the average price during the week.
With some of its new features you can automate things like buying items for de profit, prospecting. This guide goes into more detail on how to do this in the Gold Guide under the Profession section.
Another important feature of this add on is the snatch list. I use this to buy things while I run my scans that I use for crafting to de, crafting for sale, and buy to restock my bank. This add-on helps make tremendous profits but having the knowledge behind the ideas will help increase those profits.
An Auto seller
There are several on wow.curse.com (Crap Away! and Sell-O-Matic) these add-ons help you sell the things you don’t want (grays) I have heard people say “I Don’t want to pick up grays because then I have to find them in my bags to vendor them” Well this solves that problem. With some grays being worth 1-2g a piece in outlands, it adds up. Also with the expansion coming up I am sure grays will increase in price which will end up being more gold in your pocket.
Bag mods
Bag mod’s can help you from going crazy with trying to find the items your looking for. I prefer Baggins since it sorts items by type ( And I am sure there are many others out there) I am able to quickly find my herbs/enchant mats/gems since they are sorted by type on the screen. Staying organized with many auctions is half the battle. Add ons that help you keep your sanity will help make you auction hours time less stressful.
Gathering Add-On
Do you have a gathering profession on any of your characters? Then a gathering mod is a must. They save the location of nodes/herbs so you can see where you previously picked/mined some. The one I have used in the past is Gatherer which can be found on wow.curse.com.