I learned recently that there is a reason why it lists which city the retainers are based in when you're buying off the market board. If you're in Limsa and the retainer you're buying from is in Ul'Dah or Gridania, you have to pay a fee. If the retainer is also in Limsa, you don't have to pay anything extra! I've saved SO MUCH GIL since learning this. /facepalm