Auction House Price

Suggested Price vs. Real Price
When you place an item in the auction tab for sale, the auction house lists a suggested selling price. This may or may not be close at all to what the item is really worth. Some items may have a very high price that players would never pay. Other items may have a really low price that is far below the items market value. You need to determine what the "real" price of the item should be.

What Determines Price?
There are many factors so it's difficult to nail down, but these are some general guidelines:

Demand - How bad do players want or need it?
Rarity of the item - Items that are more rare will typically sell for more. Be on the lookout for various items that are sought after by many professions, such as Heart of Fire, Wildvine, etc.
Level of the item - Higher level items sell for more money
Cost of sale to the vendor - If the vendor will pay a lot for the item, players will set the item price above the vendor cost.
Auction house availability - If there are a lot of the items in the auction house, it may drive the price down.
Trade items - If the item requires rare or expensive components, the item will typically be sold for more money.
Level of the person who will be using the item (i.e., listing a level-10 item for 20 gold may not work as few level-10 players have 20 gold)

Buyout Price
Always have a buyout price. Many players want the items now and don't want to wait. Many players are willing to pay the higher buyout price just to get the item now. Try not to set a ridiculously high buyout price. This will cause buyers to say "what is that guy thinking?" and they might not even bid on your item based on that buyout price. Buyers are easily offended. It is better to not have a buyout price than to set one too high. The exception might be extremely powerful items such as purple items or items whose worth is completely unknown.

Timing
Timing can be very important for selling items. If the auction house already has a lot of the items you are trying to sell, you should wait until another time. You can also just lowball the current prices by setting your price below all of them. There are also good and bad times for auctions. A good time is when a lot of people are playing, such as during the evening or weekends. A bad time would be very late at night.

Enchanting
You can disenchant some items and then sell the resources to enchanters for a bigger profit in the auction house. You can also make more money by enchanting an item before selling it.