I'd say that sign doesn't have any legal leg to stand on. Unless the gun show management owns the property itself and it becomes a trespassing issue. If they're just leasing a building like most gun shows do, I'd consider their sign meaningless. But I'd double-check with the authorities first, or just make the transaction elsewhere. I've bought all kinds of guns, ammo, bayonets, etc. in parking lots of WalMarts, Home Depots, shopping centers, any place that both parties can identify and find.
I went to meet one seller, and the place he wanted to meet turned out to be the parking lot of the Police Dept.! He didn't tell me he was a cop, but the Uberti .45 Colt sure is nice.
I bought a Winchester from a guy and met him in the parking lot of his office building. The rifle was in his office, so we headed that way. Got off the elevator, and his office was the local FBI Field Office! That one made me pucker, but I've got a nice Winnie .44 Trapper now!
So, FTF sales between private parties aren't illegal. Perhaps the place where you do it might have an objection, but I think the gun show is just trying to puff out their chest over the issue.