Always buy on condition and not on mileage, so you're already on the ball there as far as I'm concerned. Ultimately you're just going to have to do as much due diligence as you can and go with the evidence in front of you, as even if a private seller says that they never rag the car from cold who's to say they're telling the truth? At least you have comeback when you buy from a trader, whereas it's much harder to prove what a private seller said unless you ask them to write it down.
Traders also bluff, so I wouldn't say one source is more reliable than the other. Look at the car, look at the history and look at the seller (both private and trade), and make your own call. Unfortunately there's no right or wrong answer here, plenty of people have been caught out by both sources. Do your research, go with your gut.
But don't buy from Junction 17. Ever.