20% is not out of the ballpark at all and even more sometimes. I would start with something like 30% and get down to an agreement from there. Why would audio equipment be different from buying a car, house or anything else?
This model will make the dealer go broke unless you're shopping at BestBuy or Costco.
At the retail level, high-end audio doesn't have the margins jewelry or furniture does: 500% vs 40% give or take.
And new car margins are even slimmer. The money is in the maintenance.
As for houses, in desirable markets buyers are often paying over asking price.