The one that was prominent in the news, featured in several media - was a couple that ran a restaurant. On a good day in the right season, they went over $10,000 cash deposit. The teller helpfully told them if they made 2 separate deposits under $10,000 they would be able to skip filling out these pesky forms (or she would?). So they did that.
The IRS seized the money, froze the accounts, and the restaurant went belly up because they couldn’t pay bills, rent, payroll, etc. The government said they normally don’t keep the money if the source was legitimate, but at the point the article was written, it had been several months and they hadn’t freed up their funds. (I wonder exactly who comes out ahead with seized money?) Once the press started asking pointed questions about why they kept the money, eventually they got it all back. By then it was too late to reopen the restaurant. IIRC, at one point the government offered some of it back if they would sign something admitting they were guilty and agreeing to not chase after the rest.
Note they were technically guilty, because they did split their deposit to avoid the reporting - but at the suggestion of a bank teller, not knowing it would be illegal.
I believe it was the Iowa restaurant mentioned toward the end in this article:
Let’s re-emphasize:
It is not illegal to do cash transactions over $10,000; or under $10,000.
The IRS will simply want to validate where this cash is coming from, should they be curious. (Because, of course, drug dealing and such is an obvious source of large cash transactions)
A single one-off transaction, or an ongoing legitimate business, will probably not attract attention.
If the IRS is suspicious, they hold all the cards.
But - repeated transactions with no corresponding business reason (or a suspicious one) will attract more attention.
At the very least, they will want to verify all taxes have been paid.
As the current charges at Trump Org. demonstrate, there are payroll (or gift) taxes to be paid by the payer as well as the payee, if this is income.
Except in some circumstances, very large cash transactions are unusual enough to be noted.
Just like - a 21-yo buying liquor - cool. A 21yo buying several boxes of liquor the night before high school grad celebrations - maybe worth looking into.