First of all, there's no point in a unified marketplace. Maybe for Bukkit/Spigot/Paper, but not for Sponge, as they are completely separate systems, completely separate APIs, completely separate skill sets, completely separate designs, completely separate plugins, and just generally completely separate. Ergo, you would almost never be looking for them at the same time, and thus neither the developer nor the consumer would benefit from this in any way. Communities also don't need to be unified; part of the point of Sponge in the first place was to split away from a toxicifying community.
Second, paid public plugins is a legal gray area since it's unclear whether they count as Minecraft mods, and paid public Minecraft mods is expressly against Da Rulez.
Third, your external marketplace needs to be externally moderated, and heavily, to prevent mod copies from being reposted. Otherwise you may find yourself blacklisted by StopModReposts. Being apologetic/up-front about long wait times doesn't make it more attractive either, it just reinforces the haphazard feeling.
Fourth, the above XKCD is completely true. Additional marketplaces complicate matters, and moreover aren't going to be adopted as widely as the official one meaning it'll end up being useless as a general search anyway, and increases the number of places users need to look (as opposed to are able to look).
How? If you're going to implement separate-standard developer forums too, then I suppose I'm checking in my sanity at the gate.
Not only does this provide temptation to charge for it one day, but exacerbates the potential for abuse. How does one get this "Member" flag? How easy is it?