As mentioned above, TCA membership is pretty slim at 2 rail shows. On the other hand, the 2 rail community is fairly small and most people know or have seen each other. And generally if there is a unscrupulous person around a show, the word spreads pretty quickly. Generally there are people around who are common acquaintances.
I have taken checks when I have sold things at shows. Generally if I don't know the person I prefer cash, but on the other hand, I have never had a bad check. I am not a dealer and just sell some personal items and use the table as a place to store my coat and sit and rest, so I don't have a lot of experience.