Carry it out of the case. Who ever is at the door will need to check it in. Meaning check for it to be empty an putting a plastic tie holding the action open or closed. Depending on firearm As far as lowballing go in with a price you want for it an stick to it. If a person wants to trade he will either give ya what you want or get close to it.
Leave the case in the car. It will just be in the way. Decide what the lowest $$ you will accept for it, be realistic, and don't let yourself be pressured. Be honest about the condition, but a good cleaning will help.
Look on Gunbroker to see what your gun is selling for (including shipping).
Like they said, take it out of the case. Find the most comfortable way possible to carry it, because unless you are willing to accept far less than it's worth, odds are you'll be toting it around awhile. I've seen some folks carry them slung over the shoulder muzzle-down, with an index card rubber-banded to the buttstock that has basic info and price on it. Seemed like a good way to not have to answer the same questions over and over.
I usually have tables at the Hot Springs shows -- will have five this Jan. Most any of the guys with tables are looking to buy guns, so don't hesitate to take yours.
Like they said, leave the case in the car, unless it is a special tactical case that goes with the weapon.
Have the weapon clean.
A sign letting people know the weapon is for sale or trade helps. Lots of times guns are bought and sold between the gun-show attendees without the participation of the guys having tables. Matter of fact, you'll probably get a better deal from another ticket buyer than from one of the guys at the tables. They have to buy a gun at a price that allows them to re-sell it at a profit or at least enough to pay for the tables. Some of us are not dealers, but are just looking to trade for guns we want or to sell what we have for enough to let us buy something we want and don't have.
Looking on Gunbroker is a good idea, but be careful to see what guns are actually selling for, not what people are wanting to get for them. Make sure that there are some bids on the gun, if you use that criteria to determine your price. Just because they hae a starting bid at $500 doesn't mean someone is willing to give that much.
Try to sell your gun at a table that has similar guns. Some of the guys have a particular type gun they have the most interest in and will give more for a gun of that type. Someone with several tactical weapons would probably not be willing to give as much for a classic .22 as the guy with several old Winchester or Remington .22's would, for instance.
Over the past 35 years or so of going to gun shows, I have found them to be better for just generally getting together with a bunch of like-minded guys and shooting the bull tha places to buy or sell, unless you have tables. I always welcome anyone who wants to just talk about guns and knives. I learn something at every show I attend. I enjoy being able to talk about guns and hunting without someone asking how I can shoot poor, defenseless, helpless little animals. ( I generally answer that you don't lead these animals as far as you would the robust, healthy ones.)
Come see us at the Hot Springs show. We'll be glad to look at what you want to sell and show you what we've got.