Lots of good advice already given.
*I'm always friendly no matter if I'm the buyer or the seller.
*I've been on the selling and buying ( buying more so than selling ) end of things many times. When selling I've never minded the insanely low ball price from a potential buyer .... because I have done the same when I have my buyer hat on. On several occasions , while making very low ball offers, I've gotten some really fabulous deals on items.... a brand new ( never run ) Lionel MPC set of RDCs ( 3 RDC cars, never run in box) for $50., a brand new Williams set of Powathan Arrow passenger cars ( 5 cars never run in box ) $50., Scale Lionel tank cars with pro weathering $8.00 each, a K-line scale Pullman dining car $20. new never run ( but no box. ). On the other hand as a seller I've given folks some fabulous deals as well. Hey .... we all deserve to be happy with our trains! Right? Everyone deserves to get a bargain once in a while
* Always haggle. Usually the seller can do better than the price shown on their price tag. If you don't ask you will never know if you could have purchased something you really like for a lower price.... or not. Sometimes I've asked a seller if there is any room to negotiate and I've been told politely "no". I'm fine with that answer, for at least I asked.
*As Trainroom Gary pointed out .... always check locomotives/motorized units on a test track BEFORE you purchase from a buyer. I've taken locomotives to test tracks and the locomotive was DOA.... needless to say I didn't purchase. One seller even tried to blame the dead engine on the test track transformer even as other engines were being tested and running well. If at all possible also test operating cars and/or accessories too. Don't take a sellers word for it not unless you "know" the seller and have a way for them of making good on your purchase.
* FOOD at train shows - You may want to consider packing your own. Every train show that I attend ( York, Timonium, Md., and some small community shows ) really has grossly over priced artery clogging, sodium/sugar laden crap. .... at least that is my opinion. I don't mind paying good money for good food, however, good food is not to be found at train shows ... at least at the ones I attend. ( They all do sell spring water though ) I usually pack a sandwich, some almonds, an energy bar/ water, in a small pack. ( I tend to stay long hours at train shows   I feel better physically at the end of the day after eating the healthier food. Save the excess money spent on train show food and instead buy trains ... Just saying!
* At train shows, as in the rest of life, we run into those vendors/sellers who are unfriendly/grumpy and just don't seem to want to sell anything to us. When I get this vibe from them I am quick to move on, even when they may have an item that I want. I'd rather pay a few bucks more and purchase the item from someone who has a pleasant attitude OR purchase it from someone here on the Forum OR on the Bay.
Good luck and above all have FUN at your first show. That's why we're in this hobby ... to have FUN!