I, too, was merely commenting on what we did. Some time back, my daughter went to the dealership to see about getting a new car. At the time, she had a car that fit into the 'sporty car' category. She cleaned and vacuumed it before we went. The deal they offered, while good, was not what she expected when they allowed for her trade-in. She was about to walk out when the salesman tried one more thing. He took the manager of the dealership to see and ride in her car. When the manager saw the excellent condition her car was in, they raised their trade-in amount to a more realistic amount. She made the deal.
It fit what you said, cleaning the car helped in that situation.
Since we had gotten such a good deal, my wife wondered what kind of deal she could get. I called the same salesman, and we negotiated a deal via the phone and email. She had a car that had issues. I told the salesman what we had, and he offered a trade-in amount for well above what we could have gotten for the car if we had tried to sell it. We knew and they knew the trade-in was not worth the amount they were offering. My wife worried about the car and insisted on washing it and vacuuming it before we went to the dealer. She was afraid when they saw her car they would want to change the deal. When we got there, they had all the paperwork ready. We went straight to the salesman's desk, and he started with the paperwork she needed to sign. The price was as negotiated - nothing had changed. When we asked if they wanted to look at the trade-in he said they didn't need to - that the trade-in was going straight to the auction 'as-is'. In fact, they had already moved her tag to the new car. My wife still has that new car ... of course, it is no longer a new car, and it's paid-off.
The difference was, they wanted to sell my daughter's car for a good profit - which they did in less than two days and got way more than they allowed her. They knew they would not get the allowed amount for my wife's car and had made a deal accordingly ... so they didn't care if the car was clean or had gas. They absorbed the 'loss' since they had made two sales to the same family and made a cumulative profit they were happy with.