Ok, let’s look at it from the seller’s side. For example say we have a popular unit, used, great condition, priced right among the comps.
Why do you think, as an interested buyer, deserve a discount?
Of course it's not always about a buyer thinking they deserve a discount. Sometimes it's just about having limited means. For example, knocking a few hundred dollars off a $5k amp might make a big difference to some people. It goes without saying that the seller doesn't have to sell.