I'm pretty sure I'd use an agent if I had a house to sell - they have all the connections, the skill of a salesperson, marketing tools at their fingertips, and when you look at all they do for you, it really doesn't cost that much. It's in their best interests to get you the best price they can because their pay is percentage based once sale is done, so there's nothing to lose as long as you set the price you want and don't sell for less than you're prepared to.
I wonder if the houses sold by owner are perhaps owners who happen to be in the Real Estate game and know what they're doing.