One of the main considerations for a dealer is what is the products cost. The ’margin’ on one brand may be much higher than on another. (also some companies ’frown’ on dealers giving discounts.. the dealer may lose the line it be known they regularly discount that product)
The second question for a dealer is "who are you?" Are you some stranger asking over the phone for a 20% discount" or a customer who has bough multi thousands of equipment over many years.
Then the facts of any business, Do they have too many in stock? is it actually hard to keep in stock?
Finally the personal interaction.
And yes, just like new cars dealers discounting, each dealer is totally different, and may offer a discount before you even ask, Or they may show you the door.