You can't see anything wrong in this until similar thing happen to you, if you know someone too well does that mean you know how they make their money too? Even a carpenter can be a part of armed robber group, until the day they are gunned down you will never know that they are into such thing, are you a kid or something? You should know what I am saying.
There's no need to be condescending. I made a point without sounding condescending, and I believe you can do the same. If you read the entire reply, you'll see where I mentioned the risk involved, but there are ways to protect yourself to a certain extent. I never said you can fully know or trust a person, but simply asking to know where the coin came from or how they got it.
If my friend, brother or father wants to sell their bitcoin and they come to me, I would buy. The surprise will be when they've never talked about bitcoin, or I have no idea that they have bitcoin.
You can also see where I talked about making transfers instead of cash and all that. There are risks in a lot of things we do, does that mean we should stop doing them entirely? Hence why I said we have to careful and smart about it.