First, even if you see something you like, walk away after you have carefully looked it over and asked questions, SAVE one question. Go away to the next table and look. Then go back, don't look overly interested and ask your last question, if the guy wants to deal he'll make his move then. If he doesn't be ready to leave again but tell him you MIGHT stop back then just walk away. If you find yourself really interested, stop by on your way out and ask the guy jokingly if he sold it, as your walking by (timing is important), if he didn't sell it he may be more ready to deal, remember he didn't bring stuff just to take it home. Always be willing to walk away that is what gives you control of the deal. Also, I usually spend an hour out front of the building when it opens and right after lunch (when most people first arrive) and look for "bag people" (my term). These are usually first time owners who bought a gun, put a box through the muzzle then over disassembled their gun to clean it and lost a spring or pin etc. and now HOPE to get rid of their scrap metal. I keep two 50's & one 100, if I told you what I have picked up for under $100 you wouldn't believe it. I have bought then assembled and resold (with maybe no more than $30 in replacement parts) at a good profit. I have 60+ purchases this way. You have to get them BEFORE they go into the show. I've also learned once you buy, go home because some people are unhappy once they realize there's no such thing as "just parts".
I'm sorry the Post is so long, I hope it's helpful and good hunting!
OH! one more thing, NEVER buy something you can buy at your local supplier for roughly the same price, keep your regular source open.