Would it be possible to give the guild a little more freedom and create its own "retainers" that people could sell out of and anything that goes up on the FC retainers is auto deposited back into the guild coffer ? This way the GM or any that have access can keep items moving from the FC banks thru the AH w/o it interrupting their retainers?
Also would it be possible to have a "tax" on players in a FC that draws money from mob kills on a % basis ? I have seen this in several other games, and at first people weren't open to it, but if you like a company and are getting help something out of it, and kill a lot- you wont miss the % of gil going to the guild to make it stronger.