To be completely honest here, car yard salesmen are truely eleventy billion times dodgier than private sellers.
Think about all the stories and generalisations about car salesmen being liars, dodgy bastards and dishonest... Well they're generalisations because it's mostly true. They will tell you anything you want to hear and BLATANTLY LIE to your face to score a sale.
On the other hand, a private seller also wants to sell their car, but more often than not aren't smooth talkers who will blindfold you to the truth and steer away from the truth.
I'm not saying that all private sellers are truthful, absolutely not. This is why you have to have the knowledge to back yourself up, and not rely on the car yard salesman/private sellers word about the car.
Learn to check for accident damage. Look for wear on engine bay bolts that indicate engine removal, panel removal, etc.. Look for flaws in the paint. See if the paint has been blended along a panel and look for mismatching paint on different panels... Look for overspray underneath the car on the boot floor, chassis rails and suspension components... Take the car to your mechanics and get it up on a hoist and look for any underbody damage resulting from previous accidents, look to see for any weld marks from cut'n'shuts... Check to see for built up dirt and grime under the body and suspension parts (be wary of clean underbodies and wiped clean sumps)... Get the engine compression tested and do a leakdown test if needed... Check the spark plugs to check for overfuelling, detonation, sulphur deposits from octane booster... blah blah blah...
There's HEAPS to check, and you have to do it to every car you're serious about, no matter whether you're buying to from a car yard or privately. DON'T TRUST EITHER OF THEM. Only trust yourself (or bring a mechanic friend along with you who knows what to look for)...
Good luck mate!