I bought my current car from a main dealer, I had been looking at it & comparing with similar, i watched the price come down.
I went to the dealer, spoke to him, didnt bother test driving as it had warranty etc, we sorted an even better deal. I then asked about a price for my car (which I wasnt in at the time) he offered what i wanted we did the deal, collected the following week, he took my car for a drive rouind the block. Shook hands said 'thanks; & off i went. I think comparing on line was my winner.
I also did exactly the same with a car i bought as a present for my partner, with a different dealer.
I would defiantly do the same again.
I have way back in the day bought from an auction, never had a dud.