A comment on the mechanics of this (in connection with your other thread discussing how you guarantee the price to the seller).

This is how I would structure it...

1) Seller deposits BTC to Bitcoin Brokers.
2) Bitcoin Brokers lists only 80% (20% cushion for market volatility) of those coins for sale. Or maybe it can be an estimated vol.
3) Buyer agrees to pay a fixed FIAT amount (makes it easy for the buyer). They have 24 hours or whatever time frame - this can be fed to estimate volatility and adjust the available coins. In fact, you can make the time frame enterable - the faster you can pay after clicking "buy", the more coins available for sale.
4) Buyer goes and deposits money, takes the pictures of the slip + teller's card.
5) Brokers verifies this information, clicks a button (or something).
6) Button triggers a macro that snaps/timestamps + screenshots Gox prices, and sends it through an API to the seller (or maybe just updates their account, which the seller polls).
7) Seller receives update and immediately executes the offsetting transaction.

There still seems to be a manual step of verifying pictures, unfortunately, which means you'd need enough people to cover 24 hours, or cut the hours.