I don't think there's any rule of thumb - you know all about the hassle I had with my car (

) purchased from that dealer, but others are fine.
I think if I'm looking for a particular model of car, I'd check the for sale sections on car club forums, some of those guys are insanely careful with their vehicles and they are very well looked after, and can often be picked up at good prices. Private off somewhere like TradeMe could be a hassle though.