with cars I have always managed to find somewhere online that will give you the correct “book price” for that car, at that age, with that many miles on it. Armed with this you can feel reasonably confident you are not being ripped off.
I only haggle on secondhand stuff really - but in GAK (guitar shop in Brighton) you have always been able to ask for the “best price” on a new guitar or amp and they will immediately whip out a calculator and calculate a discount
when I worked in a bookshop people were always trying to get me to give them a big pile of books for a tenner or something. I had no such authority,and nor did my manager