If you mean selling Bitcoin for fiat, it means Satoshi put his identity and safety at risk. It'll leave trace for others (such as government agency) to discover his identity and maybe even tracking his location.
Whats the big deal if his identity is made public? Even if they were to figure it out, it would be made almost impossible to prove in a court of law (plausible deniability). There is a whole legal process that any government agency would have to go through to prove he is indeed Satoshi Nakamoto, and he would be entitled to an attorney. He could just say he denies it and say that he was a very early miner and that he has been incorrectly labeled as the creator.
I get your point, analysis that makes some people believe Satoshi mined in early days and have about 1 million Bitcoin use several assumption[1]. Although i bet many people never know about those assumption. But i mentioned identity since it's clear Satoshi wish to remain anonymous.
By the way, remember that guy who actually claimed publicly that he was Satoshi? Even in a court of law under oath. No government agency has prosecuted him as far as I know.
I would guess government agency is smart enough to spot fraud. Faketoshi (such as CSW) usually incompetent in some ways (usually in technical field).
[1]
https://bitcointalk.org/index.php?topic=175996.0