You should always take gear that says it is "convertible" over the allagan coins. At level 15 you will gain the ability to break it down into materia once spirit bound form a quest in central thanalan. Even the lowest value materia will sell on the market boards for more than you would make off the coins. The main story quests will force you into the area where the quest is eventually (lv 18~). Materia is used for slotting/socketing if you are familiar with other games' terminology.