It can be a 1 owner car, and it's condition depends entirely on that owner

All more owners mean is it's more likely that one owner has abused it - but for a shorter amount of time, or one owner has cherished it and done everything that was needed and somethings that maybe weren't quite yet.
So buy on Condition
Oh and Price, Buy on condition and price
Oh yeah, Mileage, Buy on Condition/price/mileage
Oops, forgot Service History, Buy on Condition/price/mileage/Service History
And the colour you like. Buy on condition/price/mileage/service history/colour
Don't forget the options for resale and practicality though, buy on condition/price/mileage/service history/colour/options
So as everyone says buy on a combination of condition/price/mileage/service history/colour/options/time of year.