Yes, when you post an ad there is somewhere on Kijiji or Craiglists that will allow the potential buyer to send you an email.
When the seller receives the email, he has an option to respond, or not to. If the potential buyer has provided a phone number, he could call, and never give his email address out. Or, he can reply from his crapmail.com account and not disclose his email. It's just a protection for the seller against the problems out there.
I posted an ad on Kijiji a while back. Got a legit looking email from someone asking for details. I sent some details, and sure enough, I got a spam email the next day.. from "Kijiji" saying I had to login to my account to fix something immediately. Of course I looked closely and it wasn't really from Kijiji. The only one who knew I used that email address for Kijiji was the "buyer" I had replied to.