You got a lot of good answers so far. I would say bring a small backpack with something to drink. Feeling dehydrated is an issue we have had at most expos, but try to avoid carrying something in your arms or something heavy. It is usually very packed, very hot and very fun. And that's just our tiny local expo! A big purse/bag for after is great in case you decide to stop anywhere, our cresties enjoyed the cool ac at KFC after the last expo safely tucked in my bag.
As for haggling I just look longingly and stand around staring at it, works most times : ) but don't try that on something you have to have! Someone will snatch it from under your nose...
Most vendors in my experience will let you hold their animals if they arnt packed although some will say no, I have never had someone offended by asking. Also many will hold your purchase for you, saves the animals lots of jostling.