The main issue for any shop is visibility and profile. If you want to sell more plugins, then the shop needs to be a one-stop site where you get everything, where you get the newest, the best, all that you need. This shop needs to be on the forefront of your mind whenever you think "oh I would need an additional functionality, wonder if there's a plugin for that?" It must be so well known that you visit it at least once a week just to see what's new. Then, MAYBE, it would have a chance.
If the available plugins for C4D are generously spread over Gumroad, Patreon, Kickstarter, personal sites, some third party webshops, and a few obscure homepages that have been abandoned long ago (but the plugins are still good) - then you don't have the needed impact.
Naturally, it's pretty much impossible to get all developers under one umbrella (esp. since there are tax & regulation issues). I guess the first step would be to make a permanent curated list of plugins: what they do, where to get, maybe a few extras like compatibility or ratings. Add new stuff every so often, and then advertise the hell out of it (there are so many users out there that this forum doesn't reach - sadly, I have no idea how to push such a hub to everybody's attention). So far, the best I have seen is the occasional post with a plugin list, which soon sinks into the darkness of old posts 🐯
(Come to think about it, where is the info "how to sell on Core4D"? I am too stupid today to find it...)