Sarge 13 is right. The problem you are facing is very real and every amateur smart alec armed with superficial knowledge thinks he knows more that an expert. This type of arrogance will drive you nuts.
As soon as the smart alec's expectations are dashed when he receives the harsh reality that he has an "ugly baby," you have, by no fault of your own, unwittingly demeaned him. He is faced with the dilemma to distrust his own judgment or distrust you. Guess who wins that one!
The fundamental problem is that you are getting your role as "appraiser" intertwined with your role as "dealer/buyer." This puts you in the cross-hairs of distrust. Here's how you might handle this:
1) The first thing is to evaluate his expectations and establish your role. Ask the seller if he is asking for an "appraisal" or a "sale."
2) If he says "appraisal" then give him the high and low range based on the 1-5 grade scale and the uniqueness of that particular gun. IOW, educate him. Have a copy of Flayderman's handy or Blue Book of Gun Values on your laptop as a means of validating your expertise and your depth of understanding of the nuances of gun values. These other sources of value help validate your authority.
3) Be sure to balance the good points of his gun fairly against the problematic issues. Balanced evaluation is essential.
4) After giving him a range of numbers (never one absolute value) then insist that he should go to 2-3 other dealers at the show and get their offers. IOW, create an auction environment (a version of Sarge's advice). Tell him you won't make an offer until he gets 2-3 other bids. This establishes you objectivity and confidence in your numbers. The seller will now understand that you can be trusted because you are not trying to trick him into a low-ball deal.
5) After he has gotten several bids from others, then have him come back to you to negotiate. This way the seller gets a better idea from others what is realistic, rather than having you be the only one to disappoint him. When negotiating, you can offer at the low end of the range you suggested, he will probably respond at the high end, and you can compromise in the middle. Remember, other items can be bartered in the "deal," such as a credit toward another gun you are selling, etc.
The objective here is: you want to build trust by showing that you are FAIR, ACCOUNTABLE, RESPECTFUL, TRUSTWORTHY, HONEST, EXPERIENCED, SECURE, & TRANSPARENT. Anyone who can demonstrate these values will have a very high likelihood of being trusted (except by greedy or dishonest people, who you don't want anyway!).
Robert