All depends, I would never spend big money buying private, the risks are just too great (not much protection under the law like you would do from a garage). But for small money I would buy private. I have bought many cars private in my time but these days I buy new cars every couple of years or so, so im buying from main dealers now.
People talk like all garages rip you off right, left and centre, which isn't quite true. It all comes down to how you go about buying a car, check the history, get the AA or some other to do a vaild check of the car, shop around. If you don't like the salesman then walk away and don't fall for no bull of them. I nearly bought a t-jet in 2008 and I was asking questions etc and he shouted at me. From that moment I stopped the deal and walked away and I will never deal with that guy again.