A tough subject to address, I don’t think there are any set rules when dealing with a seller. In my experience dealers are a lot more likely to come off a price than an individual, I’ve had them drop the price by as much as 40% if they have had the item a long time. Others will have something for sale at a given price and ten years later they will still have it at that price.
If it’s something I have been looking for a long time and I have the money I’m more inclined to pay what they ask but I always ask if the price is “Firm”. If they are open to offers I make an offer and if they agree I buy it. Never make an offer or haggle the price then not buy it. Do not try to lower the price by pointing out something that’s wrong with the gun, just make your offer.
No matter what your interest there are thing that are for sale at many times their value either because the buyer has paid too much or believes that it’s something that it’s not. It’s a waste of time trying to change their minds. The best you can do is to educate yourself about what interest you. Buy the books first, the more you know the less the guns will cost you.