I have only purchased two boats, both through personal sales. In both instances I was lied to, in the first instance, I didn't know what I was doing and believed the guy. In the second instance I went in with eyes wide open and didn't believe almost anything the guy told me. And my instincts proved to be true.
In the second instance, I could spot obvious problems. "Does the boat leak?" I asked, knowing full well it did, obvious signs. "No" he replied, "not at all." This guy was a nuclear physicists from Ontario. Yes I did search out his credentials, found some research papers he had co-authored on the net.
This boat not only leaked from the usual suspects, like stanchion and window culprits but it had a design flaw I couldn't believe was original build. I only discovered this after the boat purchase. There were no scuppers in the cockpit and the water drained directly down the sides into the bilge. The only thing that slowed this process up was the soft bimini.
In fact some of the information he gave me - post sale - was down right wrong. I think this info was genuine misunderstandings about components on the boat not malicious intent.
So assume lies about the boat condition. And assume lies or misinformation about systems or equipment on the boat. For example, don't believe how much line and chain makes up the anchor rode, check it out yourself. Don't believe info on the secondary anchor and rode until you check it out yourself. Another one is don't believe fuel tank and holding tank capacity. Some forget then make up an answer they feel will keep you happy. And a nefarious lie some tell relates to the new auto batteries they bought and installed in their house bank just so they could tell you the batteries are brand new, this is a dangerous lie.
I like to ask folks selling things, I do this with retail sellers as well, a question in which I already know the answer too. If they don't answer honestly I now know something about them, I may still do business with them, but on critical info I might go to a second source before I completely believe them.
Create a question in which the guy pretty much has to say something negative, if he avoids answering, or ignores a glaring problem, a flashing light should start going off in your head. One question I will ask if the guy has owned the boat for a long time, "if you could go back to the builder, what would you ask him to redesign on your boat?" This is where research on your part pays off. You research and find that many who own a ..(fill in the blank).. wish their cockpit had been bigger. Or the walk way along the sides is too narrow, or their isn't a walkway on one side of their boat. Your real question to the guy isn't about how the boat is designed but more about his quality of character.
By the way, I bought the boat from the nuclear physicist, it had all the attributes I wanted in a boat, and allowed me into a marina that is very difficult to get into. I knew going in I was going to do a large refit to turn the boat into a thoroughly modern moderate distant cruiser. And the boat had decent bones, though with flaws that would have to be addressed, which I did.