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That aside, regardless of where the deal is struck, be it on the forum or AOL instant messenger, GPG would still be used to sign the contract.
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You missed the main point. Sending money to MP means losing possession of those money and GPG will not stop MP to run away it them, while a smart contract will make it impossible for anyone to not fulfill the contract since the coins will be locked and will be released only when the contract conditions are met.
Ok. And who is the arbiter on whether the "smart contract" is fulfilled? You can't have a trustless trade, and
markets are necessarily centralized.
Just so is the case with Bitcoins and markets : it makes perfect sense for the currency itself to be decentralized. It makes absolutely no sense at all for the marketplace to be decentralized. In fact, a decentralized market is about as much a market as a deenginized car is a car. The market is precisely where economic agents come together, that's what it does, it centralizes trade. Further, it is sensible for the currency itself to be as trustless as possible, and thus mechanisms that implement trustlessness are a net gain in that field. A market can never exist at all without trust, and so mechanisms that purportedly implement "trustlessness" are nothing but clunk in this field.
Do you really believe that MP will "run off" with 1k bitcoin? Or are you just trying to turn this into some
colored coin thing? I mean, the guy has paid out four times that much in
a single month's dividends!