My experience has been that the good planes sell like this, and if you know of one, you need to go for it without hesitation. I lost out on a nice Cardinal because I wavered for a couple of hours when I first saw the owner post on the Cessna Pilots Society.
I was first in line for my current 182, and though I wanted a pre-buy and more pictures after my first contact, I decided I didn't want to chance it and just agreed on the price with the owner and got it sight unseen. The owner had told me there were others coming to look at it two days later.
I did have some knowledge about the plane, as I had seen the owner post about his plane over many years and knew what kind of care he put into it. Based on that, I felt comfortable going for the plane with little more than a handshake. In fact, the owner didn't even want a deposit from me. I was on my way to Germany the next day and wouldn't be able to go out to get the plane for several weeks. He was willing to virtually shake for it over the phone. I am still extremely happy with my purchase.
I wouldn't buy sight unseen from Barnstormers or Trade-A-Plane, but I would do it again with a plane from someone I knew on a forum like this one or the Cessna Pilot's Society.
