My answer is that if it can be sold, then it must be able to be bought. My right to sell my existence is meaningless without the right for others to buy it.
Amy attempt to assert otherwise - i.e. to try to sell something which nobody is allowed to buy - would be entrapment and - IMO - immoral. Either ban both sides or allow both sides; in this case, I'd vote the latter.
Let me try putting the question a different way. Suppose there is a way that society as a whole can make some of these purchases, designed to be as fair and moral as humanly possible. Now it's possible to have individual personal sellers without needing individual personal buyers.
Now you can answer this question by itself:
Is it moral to let a multi-billionaire buy someone's life?
If it's not moral for one person to do it, then is it moral for all people to do it collectively? The outcome is the same either way (an old man voluntarily dies and his children/grandchildren are significantly richer as a result); why would it matter who made that outcome possible?
So, then:
> Is it moral to let a multi-billionaire buy someone's life?
In the scenario you've now posited - i.e. one in which society is allowed to buy someone's life - it is moral for a member of said society to buy someone's life. Why would it not be?
Yeah, but those things have a different outcome depending on whether it's a single individual or a society of individuals doing them (in that example: police being accountable to one person instead of the public as a whole).
This is different, since no matter what, an old man voluntarily dies for the financial benefit of his descendants. If the outcome is the same no matter who makes that happen, then I fail to see why one approach to doing so would be more or less moral than the other.
If anything, a single individual purchasing that old man's life would be more moral, since the alternative would be to compel an entire society (and specifically the members thereof) to bear that cost (both monetarily - i.e. via taxes - and the emotional cost of having killed someone). Given that the billionaire is (presumably) a member of society, the net impact is identical, but it's compartmentalized to a single individual who volunteered for those costs versus an entire society of individuals who might not have.
So what's your answer?