What if I buy my 10th car, and the guy I bought it from goes out and donates that money to charity or whatever the author thinks is a moral use of money? From the public's perspective, the money still goes to enhancing the public good - the only difference is that there is now a car stored in my garage instead of some other fellows.
But that is all a different matter. The relevant discussion is not about trying to prevent people from spending their money for useless things. It is about the relationship between economic value and actual utility. Any transaction for a car seems to produce the same economic value, but can have vastly different social utility. Therefore, arguments that certain behaviors should be accepted because they produce economic value which must have social utility are wrong. So perhaps it does make sense to tax the rich, even though GDP or other economic value measurements will go down, because social utility will not go down proportionally.
The price you are willing to pay should be equal to your utility of the object.
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).
The factory worker most likely doesn't care about the car. They traded time with their kids for a paycheck, to be spent on those same kids or whatever else the worker desired. Even, potentially, a trophy car they don't intend to drive.
If that car doesn't get sold then there is less work for the factory, less demand for factory workers, and fewer paychecks.