If it's something I really want, I'll ask if that's the best they can do. Most times they'll give me a song & dance and then come down. If it's not enough, I'll then state what I had in mind. They can take it or leave it. Generally we can come to an acceptable deal.
I'm polite, but firm. I do not want to insult the guy, because I want his stuff at a fair or better price. If he's a jerk about it, I walk. I'd rather do without.
I know what I want and what it's worth to me. I also know about the firearm or accessory, so he can't BS me. That gives me an even playing field.