[Dave Rifkin]

While this situation may be a little different I'd like to share my take; like others have said, we can all say "If you're not interested in buying the car then keep your comments to yourself". However, in some cases, those comments may actually help the seller.
Example; I went to look at a 1970 454 Corvette a few years ago; car looked real nice but was a slug. Therefore I was no longer interested in the car; should I have just left it at that and not told the seller why I was not interested? Same goes with the recent situation where I went to go look at a Camaro SS that I had previously owned. The car had some issues that turned me off; again no longer interested.

In both cases I politely told the seller my reasons for losing interest in the car. The owner of the Corvette did some digging and found that his vacuum advance / timing was off. Once he fixed those items it really woke the car up; he sent me an email thanking me for my honesty.
The owner of my old Camaro agreed with some of my concerns with the car and wound up lowering his asking price.

So I think negative responses can be just as productive; in some cases more productive, than all of the positive comments. This is as long as the negative comments are delivered with respect and with good intentions.