Last I heard was, a dynamic economy model has been developed for the game, and was being run in a simulator. Then kickstarter happened and, supercruise, a fundamental new dynamic developed from discussions raised with early backers (originally a different mechanic included hyperspace between points in the local system). There were also more problems than anticipated with establishing peer to peer networking and, almost as soon as ED was released, the community called loudly for (a) "more content" (bumping powerplay) and (b) more action (bumping CQC).
A dynamic economy is, by comparison to group play or combat not very "exciting", so this is probably why it hasn't been implemented yet. I say yet, because I do believe that FD would like to see the dynamic economy, as this is expected to drive emergent play, as FD injects "famine", and supply convoys / blockades may ensue.
If you notice at the moment, with the exception of rares, if you keep taking stock from a station, the amount of that item at the station does not change .. this to me screams, "holding pattern". While you'd be right, that dynamic economy would be tough to build in now, it's already built in but as of right now has not yet been activated.
That's my understanding anyway.