I think most of the time it is cash only. Although there was a seller at the TCA York Meet that more than once accepted a check from me. However, that is different because if the check was not good and the seller couldn’t resolve it with me the TCA can get involved and I could have been removed from the TCA. If you are talking about a Greenberg show or some other non TCA show I would not accept checks. Cash only.
I have seen guys pull lots of $100 bills out of their pockets. If they want your item they will get you cash.