I've found that a great deal of my wealth has come not from the items I sell but rather my timing. The only way to reliably get your product sold is to watch the market like a hawk the the moment someone under cuts you then you undercut him by 3 gil. Otherwise if you put up a ton of item at the 'standard price' that below the line everyone will shoot below of to get a sale. Infuriating but this market system is more about waiting in Agrubah then it is about your crafting skillset.