I really like Yardsellr & what they are doing connecting the classified ads of private person buying & selling with the social media space. It is a natural development, though, since it is much easier to sell & buy stuff to/from people you have some kind of relation with.
Yardsellr distiguish themselves by not sending traffic to their own site, but give tools to be used on Facebook & Twitter. Eveything is done in the feed. This strategy really moves them ahead of their competition.
Will be quite interesting to see if Blocket will move in this direction + when we will see Swedish clones?