I've always gone through an agent. It pays to do your homework - check the papers real estate and see if there are any other properties in the area for sale and try and find out when they put them up for sale - ie is the price similar to what you are asking and how long have they been on the market. Also choose an agent in your area and find out what commission they want and if it includes advertising costs.
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.
I would always use a reputable agent to sell a property.There are many very good ones here,(as well as some not-so-good!!!!) who really put in a terrific effort to get the very best price and contract details for their clients who are selling their homes.They have the expertise, and can access all the best advertising deals,so I would far prefer to leave it all in their capable hands than attempt to do the selling myself!