OK, I'm in the UK, not the USA, but in general, I haggle at most stores.

There are 3 music shops in town here. One is an "old fashioned" type: pianos, brass, violins, a few guitars etc and caters mainly to the schools market.

The other 2 are mainly guitars/ amps/ drums etc. The larger one won't discount much, if at all, but the smaller one (with more knowledgeable, and definitely much more friendly, staff) is always willing to discuss prices. The guys there know what the web prices are for most kit and will try to match that if you haggle...and they're also very accommodating if you buy multiple items. Even small things like strings and picks will get a discount if you buy multiples. And the more you spend there, the more accommodating they become. I buy all my stuff from them now, and never go into the other one.

One thing they do with second hand instruments sometimes, if a particular item isn't moving, it to "reverse auction" it: they put it in the window with a sign saying the price will drop by, eg, £10 per day until someone buys it. Generates a lot of interest from the town "musos" who try to finesse their way to a bargain. I've even heard of people opening a "book" on what price an item will go for, so even if your game of "chicken" with other interested parties fails, you're in with a chance of a modest cash bonus :-)