In my industry (financial services) I don't deal in cash. Therefore all transactions are traceable, but the MLR for us are very stringent. So a client writes a cheque for say £5000...MLR apply.
BTW suspicion of money laundering is all that is required.
So if the proverbial eye brow is raised by writing a cheque for £75,000 then a money laundering report MUST be filed. You will not know about it...but it will happen.
So, I say again, this will not stay secret.
I assume MB and other luxury brands are familiar with and not really shocked when people walk in off the street and pay cash/cheque etc. There are plenty of people out there with enough wealth from not winning the lottery to keep these guys in business. A friend who's worked in BMW, MB and Porsche dealerships over the years has said that while its not a daily occurrence, it happens.