Is it frowned upon to go into the actual gun show with a firearm and a For Sale sign in a state where it is unregulated (WI)? Should that person stay outside near the door?
I guess it depends on the show, but guys walking around with a pencil dropped down the bore of a rifle or shotgun with a flag-sign on it made out of a couple of index cards with "for sale" on it has been a fixture at every gunshow I have ever attended. It's to invite conversations from both tables and pedestrians. Just part of the fun. Standing outside hawking guns is less likely to be taken with a smile... pay your entry fee and walk around inside.
My opinion is that it's fine to conduct private sales on the show floor as long as you don't:
1) Interrupt a conversation between a buyer and a dealer to try to sell the buyer *your* gun instead.
2) If another private seller is trying to sell a gun to a dealer, don't jump in with an offer on the gun until the dealer has declined and turned the seller away.
Absolutely. There is a well developed sense of fair play involved by people who are serious collectors. Like waiting for the guy ahead of you to put back onto the table the gun that you are DYING to snap up before jumping into the conversation. If he was there first... let him look at it quietly while he decides. It's his until he places it back down. After that it's fair game.