They don't trust a smartphone app provided by their banking provider but they trust a random bloke they've never met with 10k in their back pocket. Right. Makes sense.

Scenario one - Person A bank transfers Person B $10,000 for their classic car at the car show. Person A leaves, and then several days later the money is clawed back from Person B's account as it turns out it was sent from a stolen/compromised account. They now have no money and no car.

Scenario two - Person A gives Person B $10,000 in cash for their classic car at the car show. Person A leaves, and Person B could give a shit about anything to do with Person A as soon as they are out of sight.

In scenario two, person B goes tries to use the cash, turns out it's counterfeit. They now have no money and no car.

You don't need to trust the person when they have cash

How do you know the cash isn't fake?

UV lights and checking the safety features?