Should I offer less for this new Hail damaged KZ trailer?

You have all the bargainer power in the world, whether new, used, damaged, pristine, whatever. The dealers is in the business to sell; he don't sell he doesn't make any money. You are a potential sale to him - you hold all the power.

Make an offer, negotiate, keep negotiating until you either agree on a price or you determine he won't sell it for what you feel is a fair price. Why are people so adverse to sitting down to hard negotiations?