I recently got one of my accounts banned (QQ) for doing this, so beware; there is a huge flaw in the way guild banks work.
I haven't tried this since my one toon was perma banned a couple days ago, but I've heard it still works.
1. you have to have an alt with a guild bank
2. put money into that guild bank
3. log on another character and cod something to that alt
4. log on alt with guild bank
5. take out EXACTLY the amount of gold you need to pay the cod from the guld bank
6. pay for the cod
7. log out
8. log on the character who sent the cod and collect the money
9. log back on character with bank, and youll still have the money you paid the cod with
10. go to a vendor and vendor an item and buy it back (this makes the money real)
11. repeat lol
I made over half a million gold, which is why i got caught. Having unlimited gold was like crack. However if this dupe still works, i think it would ruin all servers economies everywhere. (I alone raised the price of primals and voids on my server by 50g each.)
So have fun, but dont get caught like me.