Let’s say the absolute core is a module of shared libraries and few base classes (math, gamemode, file access, automation etc.) The first layer are absolute must haves (editor, dev UX, automation), the second layer contains the more project specific things, which you should pick if your project requires them. There is going to be overlap on the second layer.
What I understand of the marketplace is that all products sell under the same license: per Team / Business commercial usage with lifetime included updates for the lifetime of the product. I might not be completely right about it but this is what the others seems to be doing and it would make things simple. It is what I plan to do. Legal / licensing should just be handled by Epic Marketplace defaults, no surprises.
Well if I’m right about that license and if you used to work on someone elses team, then most likely the product you worked on and the plugins you used are not yours 
that would be simple enough