It’s only a slight exaduration to suggest that no one on the planet would would pay for a plugin could install a plugin that is an encrypted zip.
I have a platform that could potentially install plugins using deploy keys on sites that it manages.
Another solution would be to make the plugin publicly available but have it phone home to see if some license is valid. It could be easily cheated by someone who edited the code, but I think that’s how a bunch of WordPress plugins work.