I bought my current steed from a private seller a few years ago now. Purchased locally although I did not know the guy. Bike started life in 2007, I believe it was around 2015/16 when I bought her, I was it’s third owner and the price £250. It had been well looked after (a limited edition of carbon and alloy) some good components on her at the time. Guy told me the history of the bike, where he had got it from, how he had used it and anything he had changed on her. He presented receipts from LBS services, invited me to any inspection and I got a test ride. I suppose, looking back, it was the perfect seller and I was lucky as many people buying bikes don’t get this treatment even from shops in some instances.
Bikes still going, I’m currently rebuilding/upgrading her but I still consider I got a really good deal in my case. I think the point is, if possible, view the bike, ask questions, do a bit of research.
Just a point, the guy that sold me the bike had treated himself to a top end Cube and had no space for a second bike. Some you win some you lose.