Except that there are people for whom the price of a car is pocket change.

regardless of how the buyer feels about the cost, if they buy something that of low utility to them, then they are making irrational economic choices. But since the money is theirs, i cannot interfer 'cept to tell them they are wasting their money on useless items.

But people with a lot of money tend to spend it quite "wisely" (a survivor-ship bias, since a fool and their money is soon parted).

If you have a billion dollars, there is nothing irrational about spending 100k on a car you don't really need. Those 100k have barely any utility for you, so it doesn't matter that the car also has barely any utility.

Spending anything on anything you don't really need is irrational by definition!

Then that is a big problem with the model of a rational consumer, because no one operates to that standard, not even close.

Based on what definition exactly?