Private persons should NOT be responsible for any aspect of reporting large cash transactions. Nor should they in any way be held accountable for taking steps to avoid reporting of same.
It is frankly not a delegated power of the federal government to busy body itself with who has how much cash nor should it be u der the preview of local governments.
(Actually, we should require the States to pay with nothing except gold and silver coin at the Constitution flatly requires ... it does not matter if they call federal reserve notes money, they are not gold or silver coin and are not lawful for the States to discharge their obligations with)
Cash is necessary to Liberty.
Gold and silver coins are heavy and play hell with your pockets in any substantial number.