Essentially each transaction that is "refundable" is an implied contract between parties and it should be verifiable. Such as in the case of a car sale it is pretty easy for a third party to check if the car was received.

Or somehow have a notary to witness and verify the transaction.

This is the basic problem is that you can't have instant transfers with fraud prevention. It's impossible to claim that banks don't understand this and IMHO it should be considered criminal negligence that they pushed this product on consumers knowing that it just exposes them to easy fraud.

Incorrect. You only deserved one, not both. You don't get the money AND the car. It's either your car and someone else's money, or someone else's car and your money.