Sukebe is right about not critisizing the seller's gun. I usually ask to look at the gun.
I say something like, "This is a nice gun. I have been wanting to find one for around $400. Would you take $400 for this one."
If he says, "No way."
I say, "Okay. Thanks for showing it to me. I hope you find a buyer."
Why not be pleasant? You have nothing to lose. I've sure picked up a bunch of bargains by doing this.
Recently, I went in a gun shop. They had a consignment pistol, a 1911 Springfield with a bunch of after market gegaws on it. It looked like someone had about $1,000 in it. It was priced at $525 plus tax which put it at $568, a bargain.
I told the guy behind the counter, "I would be willing to pay $500 out-the-door." He said, "No way. No discount and you have to pay the tax."
I said, "Thanks anyway", smiled and left.
Went back the next day and there was a different guy. I told him the same thing. He said, "I'll talk to the boss." Came back and said, "I can go $475 plus tax." So, I walked out with the pistol for $514.
If you don't bargain, you never will get a deal.