This. Sort of.
I ask for what I want be it money or stuff and walk if they don't play ball. Ultimately if you're happy with the deal it's all that matters.
I walked on a 10k Golf GTI a number of years ago over £100.
I recently bought a new motorbike and it was a new model with limited availability, I still got a discount and stuff thrown in. If you don't ask you don't get.