If you've never been to a reptile show, you'll want to make sure that you know what to do, how to act, and proper etiquette.
First off, vendors are there not just to make money, but to ensure that their animals and hard work is going to be taken care of in a good home. They expect you to know a little or at least to inquire about care. Many vendors will actually have care sheets available, so if you're considering buying something, it doesn't hurt to ask for a care sheet in case one is available.
Ask questions if you aren't sure.
Personally, I'm not a fan of seeing customers dressed poorly. Now, I'm not saying wear your best Sunday clothes, but dirty, holey, and stained clothes tells me that you do not care about your appearance, and may not care about properly for the animals. That may sound snobby, but I'm sorry. Tee shirt and jeans, I'm all for it, but stained and dirty clothes is a turn off. Luckily, I haven't seen too many people dress this way.
Haggling is ok, but never try to undercut drastically. All vendors price their animals to what they feel is appropriate and what they think the animals are worth, minus a little competitive pricing. If an animal is priced fairly at $25, it may not be the best idea to offer $10 unless you're buying multiples. (By fairly priced, I mean around average for that species and morph.)
I don't mind bargaining and coming up with a price that is fair for both me and you, but I don't want to be taken advantage of.
I don't recommend talking poorly about a vendor's animals in front of the vendor. If you believe that the animals are sick or not taken care of properly, find someone in charge of the event. If you tell the vendor, it may create a scene, depending on the vendor and his/her defensiveness over the animals.
For the most part, be polite and patient. Sometimes there are a lot of people at one table, so you may have to wait your turn to speak with someone. Treat the vendors and other customers as you want to be treated.