Knowing what I know, I would have a hard time buying (investing...) in a 911 I didn't know. PCA members sell cars to each other on a regular basis as you know the car and the owner. When I sold my cars, there was never a moment's thought about advertising the car of it ending up a dealer. I had a list of enthusiasts ready to go.
So, I'm suspicious of used 911's on lots. Why are they there?
Of course, in the US, the market is different. Much higher volume (lower prices by a mile on the new ones) and a smaller percentage owned by enthusiasts, so I bet good cars can be found on lots.