I think it amounts to a roll of the dice Metin, regarding your retainer placement.
I think you'd have a greater opportunity to "own the market" (not price gouge, but perhaps be the only one selling certain things) on some items in Grid and LL, but at the same time I think the amount of traffic to the wards in Grid and LL is much less than Ul'Dah. So while you might be the only seller for some stuff would anyone just happen to be there when your retainer is there to make the purchase? You could leave your retainers in LL or Grid for a longer period, but as you mentioned in your post... would that be shooting yourself in the foot. I think it would be.
I mean if you could market the "Metin" brand or establish rapport with the player base and be known as the guy to buy "xxx" from consistently at a good price, then I suppose you could potentially draw customers to you wherever you put your retainers. Unfortunately in this game I don't think it's gonna happen like that. Word of mouth or spamming shouts is about the only form of advertisement you're going to be able to get. Start spamming shouts, and I'd think you'd end up on everyone's blist regardless of how good a deal you were selling stuff for.
Ul'Dah on the other hand is all about undercutting or buying out the competition in my opinion. There are far more items for sale in the Ul'Dah wards- both in quantity and variety. Somehow over time Ul'Dah has just become the main hub for everyone (probably because of BW and Hala leve grinds) so you will have tons of exposure to potential buyers there, but your stuff is most likely going to sell slow unless you happen to be the only one selling something, or you undercut. I don't support price inflation by buying out the competition and jacking up the price with you being the only seller, but undercutting is sucky too. Someone who crafts their stuff and sells for a fair price is prone to poor sales due to impatient people just wanting to move an item right now. You could make the argument that the undercutting like that is just here and there and that the crafters will be there more consistently so in the end they won't be hurt by it... but still at some point undercutting by ridciulous amounts hurts the crafters trying to establish a steady and fair market price.