1) Always offer your customers a fair price, both buying and selling.
2) Profit, not profiteer.
3) If you sell at the median price, the price will always come back to you.
4) A buy order is much more powerful than a sell order. A buy order is someone putting actual isk towards a purchase. A sell price can be whatever someone wants it to be. Most markets follow this rule with a few exceptions. Look to the buy orders, not the sells.
5) The more traders the better. Any time a competing trader buys, they raise the buy orders benefitting sellers. Any time they sell, they lower the sell price benefitting buyers. This competition increases volume of trade. More volume benefits traders. Few traders understand this. Stop price gouging.
6) Buy local. Support your local trade hub, it's a fallacy they're always more expensive than Jita, many times they are cheaper.