Go there with a clear idea of what you want in the gun and don't back down from it.
Having worked both sides of the table on this type of deal, I can't tell you how many people come in knowing that they are only going to buy a Colt AR of a certain vintage; but then they see the non-name kit AR you have put together and they start to sweat.
It doesn't look any different from the Colt and MAN! look at that price. I would still have $500 left over after I bought that. So they start asking questions of the dealer and depending on the dealer's honesty he either A) answers directly B) claims he doesn't know specific details (like chrome-lined barrel or who made certain parts) but THINKS that they are what you are looking for or C) outright lies to you (more common than you'd like to know).
What do I need to look for in a gun from a show, what do I need to walk away from?
If you are serious about possibly buying it, ask if you can break open the rifle and look at it. If they say no, walk away. Likewise, don't waste their time taking things apart if you don't feel ready to plunk down the cash.
First just take a look at the parts. Does the receiver say "Bushmaster" but the barrel has no markings at all on it? That will tell you when you are dealing with a "kit gun" that somebody has put together from various manufacturers. Kit guns can run just as well as factory guns; but usually the odds are that they don't.
You want a chrome-lined barrel. This means one made by Colt, Bushmaster or Rock River Arms. Luckily all three manufacturers mark their barrels. Colt will be marked (C MP and the twist rate (1/9 or 1/7 usually) and caliber). Colt may also include other markings like CB or the words CHROME BORE. These are usually older 1/12 barrels though. Bushmaster barrels will be marked B MP and the twist rate and caliber. I don't recall how RRA marks their barrels; but not all of theirs are chrome-lined so look for the silver-gray ring around the muzzle and chamber. Stay away from unmarked barrels.
Separate the upper and lower and examine the bolt and bolt carrier. Look for cracks around the cam pin hole on the bolt. Is there a lot of parkerizing gone on the bolt face or carrier rails? That indicates the gun has been shot a bit; make sure you aren't paying "new" prices. Make sure the gas key is staked on tightly.
With the gun still open, cock the hammer back and place the gun on "Safe". Pull the trigger. Hammer should stay back. Now move the selector to forward. Pull the trigger and hold it back*. Hammer should go foward (*it is thoughtful to catch the hammer with your hand as it releases instead of letting it slam into the front of the poor guy's receiver). Still holding the trigger back, cock the hammer again - hammer will catch on disconnect notch. Now release the trigger slowly. The hammer should stay cocked and not fly forward. If it fails any of these steps, the gun is unsafe. You can probably correct the problem for less than $50-80 in parts; but it is a good place to beat down the price if you are willing to take a risk. If you don't feel confident in your ability to fix the problem or pay someone else to do it, then you need to walk away because you have an unsafe weapon and a dealer who was either A) ignorant enough or B) unscrupulous enough to try to sell it.
If any of those parts sound unfamiliar or you aren't sure what parts I am referring to, take a good look at the sources at
http://www.ar15.com/ to learn the parts and how to do the function check described above. It wouldn't be a bad idea to pick up an AR15/M16 manual at the show and just follow the function checks outlined in it either.
