I’ve purchased several used cars, all found through AutoTrader. After finding a car I like, I then check the dealership’s reputation through Google reviews or Yelp.
It depends on whether you want to buy from private seller or business seller. For private seller I use Kijiji because I can use seller rating and account history to do basic screening.
I think Autotrader.ca is one of the most 'authorized' website. There you could find some dealers. And For facebook market place it's a good place if you could identify and check the car well before you got it because it's cheaper.