the 1st mistake Se made is that every character can max all classes.. they should have made it into choice's. all gathering classes , 2 or 3 max crafting classes and also 2 or 3 max jobs/classes.
Since everyone can do everything.. the market will sooner or later crash when there's no need anymore for crafters.. because we can all make it ourself.
then the only thing that still sells will be materia and having said that.. materia is a great way to make a good profit even some at tier 2 sell for good prices and it's not hard to get at all.. just do your thing and convert your stuff