As an American, am I allowed to sell over $10k of BTC to a non US citizen (he has Swiss citizenship) for cash?
If so, do I have to collect any info from him? (of course I would much rather respect his privacy)
Yes, and make sure to record and report it, failing to do so would run you afoul of AML law (if detected..)
Generally, any person in a trade or business who receives more than $10,000 in cash in a single transaction or related transactions must complete a Form 8300, Report of Cash Payments Over $10,000 Received in a Trade or Business (PDF). Form 8300 is a joint form issued by the IRS and the Financial Crimes Enforcement Network (FinCEN) and is used by the government to track individuals that evade taxes and those who profit from criminal activities. Although the cash reporting requirements apply to many types of businesses, auto dealerships frequently receive cash in excess of $10,000 and are required to comply with the filing requirements.
as i peruse Form 8300, I see you will need the full contact details of the purchasing party, and it has a space for if they are a non-citizen.
You have 15 days to report the sale, legally. this would still apply if you tried to smurf this with a series of smaller transactions.
but in my opinion, I would delete this thread, and pretend like nothing ever happened
if you go that route, be careful depositing that cash!