As someone who buys items from Guild Stores often, I find it quite time consuming to ride around to different parts of Tamriel just to find the item I'm looking for.
Why can't all Guild Stores be linked into one system where all items are viewable but still associated with the guild that is selling them? Some may think this would defeat the purpose of having location based areas where guilds sell their items. This may be true, but it doesn't mean there isn't a work around.
Possible Solutions:
1) What could be done is maybe have a "delivery fee" for buying an item from another guild using the "Global Guild Store System". This would allow users to view all items from all guilds using any guild location, but still still have a way of getting the item they want without having to search through multiple locations by actually having to travel.
2) Another solution could allow a user to view all items from all guilds using any guild location using the "Global Guild Store System", but give the location of where they can buy the item they're looking for. This would still give Guild Store locations meaning, but make it more convenient and practical system.
Users would still have the option to use the original Guild Store methods. However, there could be two options: Local and Global:
Local: View items associated with the Guild Store that's at your specific location (How it is right now).
Global: View all items from all guilds using any guild location. The name of the guild who's selling the item could be added to the UI when browsing.
All this allows users to have a global "Auction House", but still use ZOS' location based Guild Stores.
I hope other people can provide feedback and maybe it's something ZOS can implement sometime in the future.