These guns shows, if I remember corectly, had been there in 1965 when I visited the Alameda County fairgrounds. Anyone correct my memory? Isn`t this ex post facto?
No, ex post facto isn't a prohibition on changing laws or regs to prohibit something in the future that used to be legal, it means you can't pass a law criminalizing acts already committed.
IOW, you can't pass a law in 2012 banning gun shows, and have it be retroactive so that someone who held a gun show in 2010 is now guilty of having committed a crime.