Buying a used car is like lottery, doesn't matter where you buying from, but doing your research before and knowing cars obviously helps! And when you buy a car, if it's not classic or exclusive car you will never will get your money back.
I'd argue buying privately was far more of a 'lottery' than buying from a dealer. For starters, you've no guarantees, sold as seen, virtually no rights whatsoever. You could easily buy a total pile of junk and never get your money back.
Admittedly, whilst not all 'dealers' get it right, and believe me, there are some very ropey ones out there, you do usually get some form of guarantee that's worth the paper it's written on. Time and time again, I've mentioned, if you have one, pay at least the deposit for the car on a credit card, something you're unable to do with a private buyer. It gives you a whole load more rights under the Consumer Credit Act.
Yup, buying from a franchised dealership can be expensive, but they've (usually) got a reputation to uphold, so buying a used vehicle should be a little less fraught than buying one from say an independent dealer who sells any make of car. Personally, I'll never buy a car privately ever again (I've only done it once, 33 years ago, a Mini, a complete pile of junk!). I prefer the 'safety net' of buying from a reputable dealer. I realise not everyone can afford to buy from a dealership, but judging by some of the many stories I've read from folks who've bought privately, it would have been far cheaper in the long term to have bought from a reputable source.