Not sure what happened to Marshall. I can only hope he has abandoned just BGG but not Supremacy 2020.
I also didn't see his response as I haven't been a negligent FM lately.
However, before this post was made I did compile all of his answers to rules questions from his website, this site, and private e-mails, so I'll do the best I can to replace the answers I assume he had posted here:
When we purchase armies from a neutral country, what is the basis of the supply (i.e. how many can I purchase from a particular country?)
Am I limited to the amount of armies previously "sold" to that country by other players?
Do I role a dice to see how many are available in addition to what was sold to them (like I would if I were attacking, roll D6 and divide by 2)?
While I do not recall ever seeing this addressed, there is not means within the rules to place those armies on a neutral county other than by attacking. Logically they can't sell what they don't have.
What if two or more players are all trying to purchase armies from the same neutral country at the same time. Who goes first? Is it based on the same order as determined by the chit pull for who builds first?
Buying from neutrals is part of the build phase, which should be done in order of chit-pull.
Who pays the transportation costs? The neutral powers (same as if we sell to them) or does the purchaser need to pay the transportation costs (same as if we purchased from another player?)
This has been explicitly clarified - minors will pay the transport costs.
Should there be a limitation on Superpowers purchasing a neutral country's armies if that Superpowers previously attacked that neutral country (this may need to be a house rule - doesn't sound right that an invading force could buy there why out after they have already tried to invade, at least for a turn or two. Not like we could have purchased the Viet-Cong in '67 after we already stared bombing them.)
This would be a fine house rule, but is not an official rule. I suppose one could argue that a minor power might agree to disarm under duress rather than risk a continued war with a mighty Superpower (who of course could back-stab them immediately), so I think it's okay as is.
And lastly, can we purchase armies from un-played Superpowers (roll 1d6), using the same rules as if it were a neutral country (as questioned above)?
We have been playing this way, but I'm going to reread the rules and my compiled FAQ just to be sure. I'll get back on this.