I was thinking while shopping in the wards and finding out that once again Ul'dah is the only city that sells anything

perhaps we can really make it interesting by having the items that are specific to a guild have (0%) tax is sold in the city that the guild resides in.
If sold in any other city make the tax rather high I suggest 15%-25%, making it worth while for the "seller" of the item to use the other cities to get the most gil for his/her item

ex: cooking items -> limsa (0%) grid, or uldah 15%-25% more tax
items such as weapons can use this system as well

it is still possible to buy and sell items where ever you want, but for the best value, the best profit sell the item in the city that houses the Guild

I believe gear would be difficult but seeing that most gear being introduced is very specific to the class it can also be linked to guilds