I used to be peeved when I didn't get a response for stuff I was selling, until I actually started selling stuff myself. If you're selling something desirable for a good price, you will be inundated with responses. With buyers the way they are - saying they'll buy and then not showing up, etc, it can be hard to send everyone a "sold, pending payment" email. If you use gmail like I do, which gives you one way to view your messages (newest first), simply replying to the craigslist-generated reply gives the other party your email address - and if they respond to your message then it becomes a major PITA to figure out who replied to you first. So if you believe in trying to sell to the people who emailed you first, then it's actually better to not reply to anyone except the person you're dealing with right then, so as to maintain the integrity of the reply timeline.
On another note, if you ever get inundated with responses, you know you're asking too little. A fair price should always come with one or two lowball offers ;-).