Yeah, it's best to have them independent of each other. It's good enough they're linked to Kazham/Rabao/Norg. Because each city has their own separate guilds, the market there is much more catered to materials one would need. A good example is the cost of Northern Furs on Jeuno AH are like 200 gil each on shiva. In bastok (linked to norg) they're like 2k a pop.