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The most critical concept for new players to understand is that of market value. The market value of any item or service is simply the price that a buyer and seller agree upon when they trade. Average market value, then, takes into account the reality that the price will
fluctuate. It's important to understand that NPC Vendors' prices do not change, reputation discounts notwithstanding. A very common trade channel message is that a certain item is not "worth" whatever the person advertising has suggested. The concept of market value means that the item is in fact worth whatever buyers are willing to pay.
The Auctioneer addon is a huge help when trying to work the Auction House. You most certainly want to download it and use it to the fullest.
The other concept that is important to succeeding financially is that of supply and demand. The basic formula to keep in mind is that whenever the demand exceeds the supply, prices go up. When there is more supply than demand, prices go down. An easy way to describe this is if several guilds on your server are progression-raiding, but no one is taking the time to farm and fish, an enterprising player will be able to command a hefty price for his stacks of Deepsea Sagefish.