My coworker had some script or something that would alert him if something he was looking for was posted on Craigslist. I imagine some people may even have gone further and have their script email the person too. So there is a chance that within a second after it was posted, it was gone.
As for the scammers, you can usually get a good idea who they are by their email address. If they post the is it still available crap that Arkcom mentioned and they're on gmail, yahoo, hotmail, or a free webmail service like that, odds are it's a scammer. If it's from a isp like comcast, verizon, or rr chances are more likely it's not a scammer.