Look at it this way: people who buy PLEX to trade for ISK are usually after a quick ISK injection. People who buy PLEX for their account time are usually managing to scrape together the 500M ISK required at the end of their 30 day shift. Neither group are likely to have the patience to wait. In the middle are the speculators who buy from the ISK buyer for 460M ISK today, then sell to the PLEX buyer for 480M ISK next month.
If you think PLEX are sold by the real buyers directly to the real users, you have rocks in your head.