Most gun sellers know what they have to get for a gun, else they ain't much of a seller. Most gun sellers tag or initially ask some amount higher than this. It's "negotiating room", or it's "gravy money" if they sell to someone who pays their asking price. Any reasonable seller should not be insulted by an offer, unless the offer is a ridiculous low-ball offer. (He may turn it down, or counter though.)
Bottom line, what do you have to lose by haggling?
JOIN THE NRA!