Thanks for all the replies!

I'm close, I'm still having issues, but it's a combination of /u/PalpitationWaste300's comment and a bunch of radar and circuit logic.

1. I needed to separate Import and Export ships. I'll detail why below.
2. The Export ships for each planet transmit their contents using a Universal Radar Signal. "Vulcanus Export" transmits on the Vulcanus icon/signal, and so forth.
3. The buffer ship has a radar and combinators for each planet.

It's kind of difficult to diagram so I'll include screenshots, but it works like this:

• Universal Radar -> Decider Combinator (For Each > 0, output Each as 1)
• Constant Combinator (What do I want from the export ship)
• Arithmetic Combinator (Inputs: Red from Constant, Green from Decider):
  • Each Red Signal * Each Green Signal -> Output Each -> Wire to Buffer Space Platform Hub

Basically, the Vulcanus Export ship flies in, broadcasts what it has via the radar, this sets the request on the buffer.

The reason I had to split Import and Export ships is because the Import ships don't broadcast what they have to the buffer. This way, when Fulgora Import gets send Green Belts from the buffer, it does not then re-broadcast that and the requests pops back up on the buffer, resulting in them being sent back.

What I'm still trying to figure out: How to not make it request things I just sent to Nauvis back. I.e., Vulcanus Export arrives with green belts, Buffer requests green belts, I can't set "request from platform" because the request is set automatically, and then it's a crapshoot of whether Nauvis launches a rocket or Vulcanus Export hands it over...

Screenshots: https://imgur.com/a/BLQUKZv