The buyers do not approach those who have been legally deemed incompetent. As you say, such a sale would not be valid. But then again, can the isolated mentally incompetent even bring such cases to court? Vulnerable people who break a hip and can't afford surgery also don't deserve to lose their homes, especially not at a fraction of market value. There are clear gaps in the system which these buyers are exploiting and which you are repeatedly and willfully excusing. There's a chapter in the Bible about this type of behavior involving whips, overturned tables and an angry Jesus.