There are no formulas for how much anyone should pay for anything at any moment in time. Much less airguns. Much less used airguns. Far too may variables and exceptions for any formula to mean anything whatsoever. But as an airgun show regular for over two decades, both as exhibitor and "consumer", I will share some opinions based on VAST experience(s).
1) Why any airgunner would attend an airgun show as nothing more that curiosity seeker, ill-equipped to BUY A NEW TOY (or SEVERAL), is beyond me. Although my priority is thinning my inventory considerably (I'll have two tables of intensely interesting variety), just as big priority is bringing home some
new toys! Hence, we now introduce the term
horse-tradin'. Some sellers are more amenable to taking trade-ins than others, but seldom will a seller want to give
used retail value in trade. Think auto dealers.
2)
Seldom are asking prices non-negotiable. However, assuming you're dealing with a Realist, neither are offensive counter-offers not taken as
exactly that. Hence, we now introduce the term wiggle-room... and distinguish it from the term convulsions! Suffice to say insulting offers are often met with some level of obstinance, counter-insult, or complete shut-down. Over the decades I've learned to moderate my reaction to insulting offers with the simple, to-the-point phrase, "I'm sorry,
we can't do business."
3) How to know in advance of complete shut-down? Any mention of the word "
FIRM" means
non-negotiable;
NOT to be confused with "walk away". For instance, I might include some
absolute GEMS to attract attention to my tables that I'm not
motivated to sell; some possibly priced. In such cases a gun may be so incredible or unique you'll never have another opportunity to own it at any price. What to do? Ask yourself if you want to own it bad enough to pay a non-negotiable asking price; and if so, BUY IT THEN AND THERE, or you'll return to find it gone to an over-joyed new owner.
4) How much discount to counter-offer? Again, NO FORMULA. Before making a counter-offer I've already asked all my questions and/or shot the gun, and already decided the
most I'm willing to pay.
5) Any hesitation to answer intelligent questions, or any ELUSIVE answers to intelligent questions, should be taken as RED FLAGS; at the least devaluing the item, if not disqualifying it entirely. Trust me, you don't want to buy an airgun from an elusive seller any more than I do.
6) Non-exhibitors are welcome and encouraged to bring guns to sell and trade. But understand you are essentially a wholesaler, and table holders are retailers. Again, think auto dealers. And, not to be confused as a bad thing.
7) I've had more,
and more KINDS of airgun fun that anyone on Earth.
NO EXAGGERATION. That stated, the only thing more fun than airgun shows is traditionally enjoyed BUCK-NAKED!
8) NO, I haven't figured out how to enjoy airgun shows buck-naked...
9)
But I'm working on it! 
