Let's walk through the 9. The 3 nominally liberal justices are all too young to retire before the election. Breyer already made the choice to not become another RBG.

Of the 6 nominally conservative justices, the only two old enough to retire because of age are Clarence Thomas and Sammy Alito and even then justices regularly serve into their 80s. Mid 70s just isn't going to cut it. Besides, either will resign unless there is a conservative in the white house. The only possible way for one of the conservative justices to resign is if one of them gets impeached and it looks like he is going to lose. Even then, in this climate they would rather make a political circus of it. As it is, there is enough credible reporting that in a different era a few of them would have resigned. See e.g. Abe Fortas. Since they haven't, I don't think anything more would push them.

I was thinking more along the lines of a voluntary/semi-voluntary decision, rather than a death or an administrative removal like in an impeachment. I would resolve death or another possible outcome to N/A.

How does this market resolve if one of the Supremes dies? I imagine the point here is the political impact of a vacancy in the Court, and for that it doesn't matter how exactly a Justice left office, right?