If you are using B to circumvent A on the front lines of a battle, or somewhere similar, that will, at the least, be deserving of much greater scrutiny. Please don't do it, and if you know of it happening, contact the Titans.
The guildhouse loophole is allowed for three main reasons (at least in my mind): First, because it relies on being in a specific location that has been prepared ahead of time, second, because it requires those exchanging money to be full members in a specific guild (which can be dealt with relatively quickly, but still provides some barrier), and finally, because the guildhouse's treasury is, in fact, primarily intended for the guild's full members to use to store and exchange funds.
The ability of adventurers to buy and sell unique items is very clearly not primarily intended as a means of transferring gold between nobles in the same region.
