That stupid message has been a thorn in my side for a few months now. I'm the guy that put this site together a few years ago and still help Gar out when he needs a hand from time-to-time. Working on this site for Gar is what got me interested in pre-war baseball cards.
That said -- and I apologize for the techno-babble -- something keeps injecting some Javascript into the main page of that site and that is what's triggering that warning. I've 'fixed' the problem half a dozen times. It's on my to-do list to fix it again tonight and contact the host again about preventing it from happening. What's especially annoying is that if you visit Gar's site through the other [primary] domain:
http://www.baseball-cards.net you don't get the error/warning even though you are literally viewing the exact same page/file.
Long story short, visit
http://www.baseball-cards.net and you won't see that problem.
Regards,
Richard.