I think what they are saying is that the counter-party in the in-person BTC/USD trade could have gotten those USD in an illegitimate way, and that by accepting those USD in exchange for BTC you’d be partaking in money laundering.
Is there really a legal responsibility placed on an individual selling BTC for cash to verify the source of funds?
I assumed that wouldn't be necessary if not operating like a business with the intention to turn a profit. If I sold my car for cash, would I have the same responsibility?