You can deposit a check in any amount without Fed notification, cash over $10K gets reported but you can walk in one day and deposit $5k and the next day another $5k without any issues. Heck you can deposit $9k a hundred days in a row if you want.
The $10K limit was to stop drug dealers and others involved in illegal activities from depositing many 10's of thousands at once.
There is no need to
By law, if the cash deposits are linked to a single transaction they have to be reported no matter how you break them up. Consecutive daily cash deposits of 5k or 9k would certainly trigger a report.
I don't see what this seller is concerned about with a check for the proceeds.