There are alot of trends you have to look at:
1. What are you trying to sell, is it a skill up item? If your trying to sell an item that someone or some people are skilling up on atm, that the supply will be high and the price will drop. Also, if a group is skilling up the materials to make the item will be in higher demand and drive up the cost to make an item, while the item it self is over stocked driving down the breakeven. Crafting is about the +0.2 than about making money at times.
2. Flavor of the month: What are you trying to sell and where does it come from? Sometimes people figure out a BCNM or the location or pop time of a NM becomes common knowledge and the market is flooded with an item.
3. Does the item your trying to sell have multi-use or is it only a exp item. If your trying to sell level 30, 40 or 50 gear, you may have hit a trend in CoP mission where people are dumping their gear after finishing up a chapter.
Really you need to watch the AH, yeah at times I have listed one item at the normal selling price without any other item listen, only to have the item returned and seeing 10 other sales that undercutted the normal selling price. The best thing is to wait to sell when items are low in stock and try to buy when items are high in stock. People will pay if there is only one item and they need it, but when there are 20 items expect people to drop the price cause they want their money 1st, no matter if it is at a loss from what they paid for it.