As an example, I know that prices for Goblin Glider Kits fluctuates a lot. I used to sell mine for around 30-40 gold each about 2 months ago, but recently the market was flooded with them so the price is down to under 10g each on my server. By experience I am pretty certain that the prices will go up again, so right now I am hoarding them on one alt, both by making them myself and by buying them on the AH when they are really cheap. As soon as the price goes back over 20g I will start unloading them for a profit.
2. Once you have an interested buyer, stay firm in your price. This does not mean to never make them feel like they are getting a bargain. You can subtract 5% off the price and not be hurt as long as you still make a profit! Remember this, as that 5% may not be a big deal to you, but the buyer will feel special and might buy the item for the slightly reduced price. I can not emphasize enough that this is not the same as undercutting! Do not drop every good every time or you develop a reputation as a soft seller and people take more liberties in their dealings with you. If you give someone a constant 5% discount, then they begin to want 10% and then 15% until you no longer make a profit. Discounts are used when you feel that the seller may be pulling away and you really want to just profit and move on!