Always post a buyout price on your auctions. Don't think that "They'll just bid it up anyway." Many players will not bid on an auction with no buyout price unless the item's bid price is heavily discounted to begin with. This can lead to bidding wars, but in many cases the item will sell for a fraction of what you could have gotten if you had posted a buyout price in the first place. You will have more sales at a higher price and get your money more quickly if you post a proper buyout price.
Soloing an instance can be all the more profitable if your character is an enchanter. During any solo instance run, you'll end up with some items which can be sold or traded, and some that are bind-on-pickup. Non-enchanters can only sell these soulbound items to a vendor, but an enchanter can disenchant the bind-on-pickup rare items, and then sell the shard/dust/essence instead of just selling the blue item to a vendor. (A side benefit of selling enchanting materials is that they do not require a deposit to list in the auction house, allowing you to list endlessly until the item sells.)
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.
Different qualities aren't limited only to wild pets—many rare World drops and special non-wild companions have had their stats boosted. For example, the world-drop dragon whelps (Azure Whelpling, Crimson Whelpling, Dark Whelpling, and Emerald Whelpling) are of [rare] quality. Check your Pet Journal to see the color-coded rarity of your non-wild pets.
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.