One thing I have found useful is the the concept of "crafting bottlenecks". For each craft, there comes a point where the raw materials are no longer sold by the guild supplier. To advance, the craftsman either needs to gather or craft these things themselves, or search for them on the market board. Some bottlenecks I have experienced are Iron Ingots, Aldgoat leather, Diremite Webs and Fish Glue. There are also plenty of Culinarian bottlenecks, which require materials a good Botanist or Fisher can easily supply. Just the other night, I sold a stack of 99 Ruby Tomatoes for a nice little profit.

Another thing to keep in mind is that, if your unit price is high, you may benefit by selling smaller stacks. Most lower level players are chronically short of cash, and are more likely to spend 5,000 for a stack of 25 200-gil Iron Ingots than 19,800 for a stack of 99. (I call this the "Home Depot" effect -- if you're going to Home Depot, you probably don't want a 25-pound bag of nails.)