Yeah, I've just seen so many "damn, it's sold!" posts after an initial post like this here, it makes you wonder? If nothing else though, good bikes at a fair price move fast on Craigslist in and of itself. You might wanna just email the owner again to emphasize that you are a serious buyer and to please hold it for you. You of course would be slightly giving away your hand to negotiate his asking price, but the main thing is to just give yourself the best chance at getting the bike you've been waiting for.

I have a feeling you'll be okay though. Check back tomorrow with your new steed.