I've been trading in MMO's since I first started playing them. I remember my first Phat acquisition in Runescape and then the quick exchanges that led me into great riches that seemed to follow subsequently post haste! After Runescape I played the markets on several games... FF online, Knight Online, LOTRO, Summoner and Runes of Magic just to name a few. The fundamentals are all the same with few differences with the exception of the means to which we trade; let me tell you... Wow has come a long way in terms of a diverse and easily manipulated marketplace. This is assuming you have the tools to really take advantage of what is in front of you.
I'll stick to a few of the tools in this first post and maybe a couple tricks to help you maximize your trading experience.
First up, Auctioneer. This addon is a classic, it came out originally in 2006 and has developed into something quite extraordinary. The most useful feature of the program is it's ability to track other posted items and while storing this information it recommends in your tooltip what a good price to auction your item would be! After first installing the addon, you should open up your local auction house and let Auctioneer do a "scan". It will take several minutes but when you return you'll notice it's completed a scan of every single item currently available on the auction house.
So how does that help you? First off, it's going to take into account different ways you can get rid of an item for profit... namely, "vendor cost" "disenchant cost" and "auction cost". This is particularily helpful when you are farming old world instances and don't know whether to sell that soulbound ilvl 350 purple to the vendor for 12 gold or disenchant it to a Maelstrom Crystal to sell on the auction house. (hint: vendor)
Auctioneer offers a more robust search engine for searching the auction house, allowing you to set multiple criteria's and even bidding rules that your search must adhere to in order to filter out a lot of the nonsense items you otherwise wouldn't want to look at.
Immediately you should also notice a new column on the right side called PCT. This is basically a percentage of what the item you are looking at is currently listed for compared to the price your auctioneer has seen the item at in it's history through multiple scans. If you can imagine the first couple days it's not going to give you radically different values because it needs a bit of data to benchmark the real value of an item; when you've established yourself over a couple weeks of playing the auction house though those low pct items can turn for great profit in a very short amount of time.
Anyone serious about professions, gathering/selling goods or simply trading should own Auctioneer. http://auctioneeraddon.com/ This is the developers site but you can also find it on curse.com.
Next to Auctioneer but along the same lines is one for you iPhone users out there! Wow armory for the iphone is a new addition to my arsenal and I can kill hours and hours a day checking my auctions using this app on my phone on the go! Not only does it allow you to check your in game mail for bids/auctions and sales but you can see information pertaining to all of your characters as well access to your bank items. All day long you can post and retrieve bids from your phone which allows for less micro managing at home at the end of the day when you want to get into some instances. This app for your phone is free and totally worth the few minutes to check it out.