I couple of thought touching on things mentioned above.
Know who you are dealing with! There are dealers who have been around long enough that you recoginize their faces or, your friends do! These are the guys that will talk straight with you. You may not get the absolute lowest price at the gun show from them but, you don't have to worry about getting taken.
Remember, at a gun show you are generally a small fish in a large pond. If you don't know what something should cost, how it should look, how it should feel, you are suscpetable to scams from the less reputable fly by night rip off artists posing as gun show dealers.
I find it really isn't necessary to spine whack or other wise seriously (or is that abuse?) a dealers knife. A first quality knife from a known manufacturer will take it, a second or cheap import can be easily spotted so why waste your time destroying somebody's piece of junk? Does the blade wobble, does the liner meet the blade properly, etc. can be easily done with a visual inspection.
When you walk up to someone's table, look it all over? Would you buy a Sebenza from someone who had 80% of their table loaded with cheap imports? Generally, you are going to find a dealer either carries good production stuff (i.e. Spyderco, Benchmade, etc) and high end (CRK, Mission, etc.) or they will carry a ton of the cheap stuff with some of the better production pieces. If something looks like it doesn't belong on the table or doesn't fit the overall image of the table, walk on or be very wary and take your chances.
Finally, you will find many fantastic people at the gun shows who are selling their wares to supplement their main job and are not out to scalp anyone. They just want to have a little fun on the weekend and make a little money for their efforts. Patronizing these people will you real happy. As for the other elements who are your classical hucksters looking for a quick easy buck, steer clear!
Knife dealer, FFL gun dealer and, gun show patron since I was a child.
I am a dealer for and stock:
Mission Knives & Tools, Inc.,
Tactical OPS USA from Idaho Falls, ID,
Chris Reeve Knives,
the Krill Lamp from Kriana Corp.,
Underwater Kinetics flashlights and,
with other fine products being added as they are found and time allows