Was looking around searching for scams, all the guild scams seems to be reposts, but I did not really find anything about this, some may find it useful, other might see it as common sence, but since most other scams, you simply exploit peoples stupidity.
First of all, make a guild, supposidly with some kind of bank name (e.g. Bank of Azeroth), pay for a guild bank.
Now, start recruiting members, say that you are hosting a bank for everyone to use, that their bank characters will be offered to get a bank usage with a certain ammount of slots.
As even one bank is devided into a certain ammount of sections, you can convince the victim that he will be the only one having access to that special section.
Simply keep hosting the guild bank and get money from it, when there are some expensive items in the guild bank (gems or cards etc, that people might put in), simply remove the victims access to it (so that he can only deposit, not withdraw), hopefully it will take some time before he notices this.