This is a really interesting topic. I have to admit, I haven't put too much thought into the specifics of the system - but ideally, it *would* be self-regulating. Of course, there's a pitfall there too - if it's really good at regulating itself, you might have a tough time influencing it as a player.
As far as supply and demand - it's a good starting point, but it gets tricky trying to figure out out future supply and future demand. Also, ideally you'd take location into account - it does an outpost no good if there's no fuel anywhere around for light-years, regardless of the fuel stockpiles elsewhere (which may well be being hoarded for an artificial price increase). In fact, the fuel could be right next door, but owned by a hostile faction. And it could be bought by a neutral merchant and sold to this outpost, so it's not strictly unavailable.
Then there's a question of how often you update prices. If you don't do it with every unit of something sold by the player, you could have a situation where the player can buy/sell the same stack to the same planet over and over, with profit (costs X: players buys, there's less "current supply", it costs >X, player sells, repeat). That's not particularly hard to avoid (most likely with "hacks", whose effect on the overall economy may not be immediately apparent).
You could punt on a lot of specifics and boil it down to "simply number crunching", but the question is, would that produce a good enough system? My gut feeling is you at least have to take locations and faction relations into account before it does. Maybe level of hostilities in an area, too.
So... well, this bears a lot more thought. I certainly wouldn't describe supply and demand as "simple", though. In the meantime, I'll be watching this topic