I always thought cash was king, however....
Had a dude rock up to buy a car I was selling, said he liked it and wanted it so put down a $2K cash deposit there and then, then came back a couple of hours later with another $15K in cash, in a literal brown paper bag. I thought great. We exchanged all details for the transfer of ownership and registration. He drove away with the car, I walked away with the cash.
Following day I rock up to a car yard to buy a new set of wheels. My payment method was going to be a combination of transfer of funds from my nominated bank account and the $17K of cash I received from the sale of my old vehicle.
When I signed the paperwork for the new wheels and placed the brown paper bag on the counter with $17K of cash, car sales guy freaked out and didn't want to know me. From memory he mentioned there's some limit now ($5K?) for cash transactions due to people laundering money. I was like WTF, it's cash mate, surely I can pay for a purchase how I like? The sales manager got involved, made some calls and eventually produced some document from the ATO I had to complete and sign with details of where the $17K had come from. I was also advised I may be questioned or interviewed by the ATO at a later date if the story didn't match up.
Never heard anymore, don't know whether the guy who gave me the cash was investigated, but at the end of the day dealing with cash sales and transactions is no longer what it used to be.