Let's face it, trading guilds are nothing more than random invite guilds made up of strangers with zero intention of ever interacting with each other outside of trading, so why among 400 strangers does 1 person have all the power over the other 399 players?
Guild masters of trading guilds have a 500 slot bank storage all to themselves!
I just joined a trading guild for the first time to see what it's like. What it's like is more a storage system than an auction house. Blue items were ~1k each, purple items were several thousand a pop. All the vendor trash was priced in the hundreds.
Then I take a peek at the guild bank... where everyone has deposit rights, but only the guild master has withdraw rights. There are dozens of racial motifs in the guild bank, motifs that are listed on the store for 1k+. Motifs that can be bought from zone chat for less than 500.
I'm not fully aware of all the powers a guild master has beyond the obvious control of the guild bank and the membership of every person in the guild, but to me, that alone is too much power for a single individual in a random group of people. Spam random invites for a trading guild and receive a free 500 slot storage bank!
I know, if you don't like it, no one is forcing you to join a guild. That's true, and that's why I've not done so until now and promptly left after seeing how they operate. However, without a public AH, you can't really say that a person isn't at least somewhat compelled to join a trading guild.
I think a simple solution would be to assign ownership of guild bank slots to the individual members of the guild, with each individual having complete control over their slot.
Just an idea... and now I'll go back to pretending trading guilds don't even exist.