John, You are right that selling privately is different. It is illustrated by your statement:
"I don't care if someone doesn't buy my car."
If your livelihood depended on selling the car, I think you would care. A dealership cannot take the attitude that they don't care. They have an obvious stake in making sure that their efforts in maintaining their property, supplying stock, hiring staff, etc. results in a car being delivered. That is why they will ask for a deposit when delivery is not happening immediately. Unfortunately, in this day and age, the word of strangers cannot be trusted and getting money is the only thing that will affirm commitment in most people's minds.
You are absolutely right that people shouldn't impulse buy a car and that they should be prepared to buy if they leave a deposit. It uis not the car dealers place, however, to read the mind of somebody they met only a few hours ago and determine if they really intend to buy a car or if they are just going to change their mind in a few hours.
Now, asking for a deposit in the middle of negotiations is a whole different issue. Please note that I am only commenting on leaving deposits after a deal is negotiated.
