> Luxury cars in particular are routinely bought and sold for cash.
Cash? As in hundred dollar bills?
Cash? As in hundred dollar bills?
But it doesn't take too many cash purchases that are not inline with your tax returns before somebody is going to start snooping around.
edit: although sometimes, a lot longer than one might expect.
Citation needed? Where did you hear that this is a routine occurrence? That seems risky for everybody involved, and it requires a report to the government from the seller.
A dealership may take issue with it but a private party accepts no liability by taking cash.
Secondly, while it's possible to construct a private party transaction in the US where this is fine, if the person spending the duffel bag acquired the money illegally, you the seller are liable if you should have known. "Willful blindness" when accepting illegal proceeds makes you liable too. See 18 U.S.C. § 1957
Maybe I misspoke by saying you would be liable for laundering specifically, but certainly accepting that money is a crime if you have any reason to think it was ill-gotten. And that's a huge risk that no one wants to take on.