How will you enforce a fiat transaction?
The deals are personal. I will not be personally (if the question was directed to me) enforcing anything. It is a personal transaction between the owner of the web site and the person interested in interchanging something, whether it be currency or goods and services. Since it is a local deal, the interstate commerce thing as well as the international laws are moot.
Say I sell 1 BTC for 30 USD, suddenly the MtGox price crashes and BTC is at 10 USD/BTC - how do you ensure I still get the 30 USD somebody has committed to?
Again its a personal deal, like in the offline world. It is not going to be connected to outside influences.
To me the small hints you gave seem like a "mobile"(?!) version of bitmarket.eu or similar, where you can put up offers that might be taken or not - but every time the BTC prices fall there are lots of problems with people not doing the fiat transaction. On the other hand, if you don't even hold BTC in escrow, as long as the rate goes up it makes sense for sellers not to send the BTC they promised but rather to re-sell them at higher prices.
If there is going to be a terms page on the hub site, it will say that people take responsibility for their own interchanges.
Regulatory issues come into place once you start to create something that is 1:1 pegged to a fiat currency and can be redeemed in fiat again. This is the main difference to gift certificates (e.g. iTunes gift cards). Yes, of course sometimes it might be possible to get money back on a gift card too, but in general Apple does NOT guarantee you that you can just show up with 5000 of iTunes gift cards and demand 10 500 bills. With e-money on the other hand, you could do that.
Nothing is pegged to anything, as all interactions are done "person to person".
Its total freedom. It does not involve gift cards, just regular digital data running through the internet. HTML, PHP and perhaps Javascript which are all "open" technologies, are the powering influence.
