My rules of negotiating:
1. Always know what a comparable item would cost somewhere else. Be sure to figure in FFL transfer (if applicable) and shipping, but keep in mind that by buying out of state and having it shipped you generally avoid sales taxes. When the gun costs $750+, shipping and FFL fees are generally less than the sales tax where I live.
2. Convenience, good relationships and being able to inspect before buying are worth something. If local shop has something I want, I will generally be willing to pay $25-50 more for it there as opposed to over the 'net for those reasons.
3. Always have a ceiling price in mind, as well as a "good buy" price. Your objective is to get the item for as close to the "good buy" price as possible, but you should never exceed your ceiling.
4. Never "ask" if the price is negotiable, or if the seller "would take" a lower price. That just invites the "no" answer and makes you appear weak. Instead, you make an offer. Just tell him what you're willing to pay (which is generally somewhere between your "good buy" and "ceiling" prices. If he says he can't do that, then put the ball in his court. Ask him what he can do.
5. Always be willing to walk away. If not, you'll pay too much.
FWIW, shops around here will ORDER anything for 10% over their cost, plus shipping. That's because it's a guaranteed sale. If they have to inventory it, their costs are actually higher (it ties up capital sitting there in the case), so they have to price it higher. The longer it sits there, the more expensive it is to the shop. No shop can afford to price things in inventory at 10% over their cost, or they'd go out of business quick.
Finally, when something has a good price on it to begin with, I don't dicker to try and get a stupendous deal. When I came across a like-new Arsenal SA-M7S the other day with a $399 price tag on it, I just said, "I'll take it." As the old proverb says, never look a gift horse....