Without doing any research he heads on down to the car lot and hooks up with a sharp salesperson. Joe Consumer might be talked into buying a car in too high a price bracket. He could also be switched to leasing without his knowledge or consent. And he might also put down too much money. This might convert into hundreds of dollars too much per month.
Many people are on tight budgets.
The monetary loss is only part of the problem here. People become angry, humiliated and resentful once they find out they have been deceived and overcharged. I should add that there are many good dealers who wouldn't cheat or overcharge even a naive customer. But, sadly, there are enough unscrupulous dealers out there to make caution and suspicion necessary. I know that many of the salespeople I worked with would take an extra thousand dollars profit without a thought, then laughingly brag about it to the other salespeople.