If Plex sells any kind of digital content, it would certainly have to be some kind of licensing deal where Plex was the middleman for an existing firm’s library. I cannot see Plex building their own equivalent to the iTunes movie shop. That would be a huge job.
If I want to buy a digital copy of a movie, where can I go today? Apple, Google, Amazon… Who else?
I cannot imagine any of those enormous companies cutting a deal with Plex to let them resell that content. If there is a smaller company out there offering the same kind of service… Then maybe.