The number one concern I can think of is security - a corrupt person making it into the system at a high level, or as an Admin, could easily thrash the system. So I propose (and GBM has also mentioned) a redundant system - a separate, non-forum-based accounting system. I am imagining the Grand High Bankguy running a database on his/her comp in order to keep track of the transactions, and reporting balances out to the admins regularly for updates. The transactions should be listed somewhere reviewable - the handling of currency generally necessitates audits, you know. This way, if there is a sudden balance change not accounted for by large transactions, the Admin will know the banker is being fishy, and if the balances change more than the Banker can account for, the Admin has been naughty.
Collusion between and Admin and Banker, though, presents a problem, unless everyone else keeps diligent trecords as well,a nd I don't forsee that happening. So do we include a Governmental oversight, as well? So many questions.