I'm 100% sure you can, as long as your not copying open source code from other plugins that don't allow for building off of them with a commercialized product.
Plugins completely disappeared since v0.8.1
Since I've updated from v0.7.4 to v0.8.1, a lot of plugins have disappeared from the NodeBB plugin list, "nodebb-plugin-emailer-mailjet" included, which I was using for email dispatch.
Also I remember that the plugin list was at least double of what it is now.
What is the reason behind the disappearance of so many plugins from the "Install Plugins" page?
Only plugins that are updated for 0.8.0 will show up in the list to prevent people from installing outdated/unstable plugins. In your case
nodebb-plugin-emailer-mailjetneeds to be updated to 0.8.x.
See https://community.nodebb.org/topic/6298/nodebb-v0-7-3 for more info
Thanks, I'll try to communicate with the plugins' developers