As a former Porsche used car dealer, I can tell you what I used to look for.
Low mileage cars with complete documented service history.
No signs of abuse.
No body work
No non-oem options
No modified or tuned engines or suspensions
Perfect or near perfect cosmetics.
No funny business on the title.
No funny answers from the owner.
No stories of any kind.
No ricer add ons of any kind!
Now, I was coming from the perspective of having to sell the car to others, so I was pretty clear what THEY were looking for.