Lots of good points in the replies.

From them I've learned
*know the value beforehand.
*both parties probably already know pretty much how the negotiation will come out.
*dealers often sell for twice what they paid. As I left I asked if he had about $100 in the pistol. He said "A little more than that."
*a successful negotiation without rancor may lead to a good deal in the future.
*don't obsess over not getting it $10 cheaper.

Thanks for the advice. Here's a picture of the pistol.