Ok so like every solder needs his weapon to do his job right that's about the same with everyone that wants to make gold in WoW but instead of a weapon we utilize addons. Here i'll try and give you the run down of the essential addons you'll be using on your way to becoming a millionaire WoWer, so lets get started.
TSM (Trade Skill Master)
This addon is your bread and butter when it comes down posting large amounts of items and quickly checking the AH if someone has undercut you. (Do know that it takes some time to understand how it works but its time well spent learning it)
One of the best addons that deals with the auction house and mass buying and selling of items
About the same as the one above but for some reason some people prefer it to Auctioneer but i say use em both.
This is the addon that makes mailing huge amounts of items from one character to the other not being a pain in the ... back.
This is what you use when you get 1200 mails in your inbox but you don't want to spend an hour at your mailbox collecting them.
Not as much as a gold making addon as it is a gold tracking addon that gives you a run down of what you've sold how many and for what prices.
This addon will either make you love it or hate it, but if you like making a stock of items and keeping it up for continued sales that this addon will make it so so easy to keep track of what you have and what you need to craft in order to have it in stock.
Last but not least is this addon that will make your job (disenchanting, milling or ore crushing) a fast and easy task.
Those were the basic tools but i'd like to give you a few more sites that i've personally used and loved to help me dominate a market or a competitor in a certain market.
The Undermine Journal or for our US goblins, as for people that play on servers that are not on English language they can check out wowTrader or AHSpy
So there we go this should be the bare minimum you have for addons and sites that will keep your needs for auction house information and profession needs sated.