I've not responded before, and for several reasons. One is there are a LOT of ads on Craigslist and other sites that offer stuff that's no longer for sale, even when they say "If the ad is still up, it's still available". So I may email two or more people offering an item I need, and when someone replies first and gets my business I don't bother to return the emails of anyone who contacts me later.
Another reason is I may see something interesting and email someone about it, but then rethink my interest in it and decide, for one reason or another, to stop pursuing it. And when that happens I usually won't respond to any emails I get.
And then there's the people who require you to contact via email, but apparently never check their email. Then, you get a reply a month or two later, and by then you've already moved on.
Lastly, there's the total dick guy who replies with nothing but attitude, and I never waste my time with those sellers.