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Addons are one of the most important things about using the AH. They provide an easier time of posting your auctions and finding amazing deals. They're pretty easy to install. There's a guide here on Wowhead if you need help. The following are some of the addons I use.
Auctioneer is the best addon I have. It does scans of the AH pages which can take about 15-25 minutes. Then you can search through the data that it collected and purchase the cheapest things. It's a really handy addon to have even though it is old.
TradeSkillMaster is another amazing addon. I would go for all the modules since they all seem to be pretty helpful. I only chose half or so because I already had addons that did things that some of the mods did. The main mod that catches my attention is this one: Accounting. It keeps track of all of your gold made within the time you first got the addon and the current time. I'll show a screenshot below at the end of this guide.
MogIt seems odd in this section doesn't it Well in this case it will be very helpful. MogIt shows what an item looks like when you mouse over it in your inventory which means you can identify what looks good. What that means is that if your item looks good then you'll get profit from it because it looks nice and people will be willing to pay more then usual.
Ark-Inventory is a must have in my opinion. It is extremely helpful. It provides a different interface where your bags are all joined, it's much easier to use, you can sort things, and you can search for things.