I have haggled at gunshows before. Rarely do I ever go to a gunshow with the idea of buying a firearm. Usually to buy reloading supplies.
If I do haggle, it is on milsurps. Often I see a gun I want and ask if the price is negotiable. 80% of the time it is not. Thus I walk.
I have had guys ask me how much I will give for it and quote a price I am willing to pay imediate cash for. If they accept, out comes the cash. If not I walk.
If I really am interested in something, I go back to the show Sunday a couple hours before the show closes and ask again. It is then I sometimes make a favorable deal or more often, walk away.
Agree with Rifleman 1776: cash on hand is king.