Any bank (Square icon) in any major city will allow you to buy from and sell with any guild that you are a member of.
The guild store you bid on is for people in the game who are not in your guild. They place the bids in veteran zones because there obviously is more gold in selling vr items to end game players than low level stsrting zones.
For instance, a trader in Daggerfall is open to all daggerfall covenant. But it is also available to ebonheart pact members when they are in cadwell's silver. Members of the aldmeri dominion can access it when in cadwell's gold.
That's how I thought it worked it anyway.