Well, space trading has certainly gone nuts since Elite, with the
X series being the most robust (and frustrating and slow and...)
I myself am working on a small trading game (not like Elite, however), so I have been thinking about trading game mechanics recently.
I think the core of the appeal of trading games is simple and tycoon-like: it's a pleasure to start small and end up big. In typical RPGs, this progression is earned through fighting: fight, get XP, level up, be more powerful. In a trading game, you trade goods, get $$$, which you can use to expand your capabilities.
A good trading game will reward the player for being shrewd (again, in a typical RPG the player is rewarded for sound tactical decisions in the moment of combat. In a trading game, a grasp of the big picture is rewarded). This is the most difficult aspect to a trading game. You want the system to be clear enough that your players can understand how things work, but opaque or dynamic enough that they need to think about how they can make the most money. That way, when they do find a very profitable route (oh! Gamma 9 makes rayguns which means they always need a lot of zinc alloy to build them with! I should sell all my zinc alloy here). Dynamic events that change the flow of trade is good: if the player can influence them, even better.
Also, all that money should be good for something -- expanding your trade empire should be fun in its own right.
And, as in most games, introducing difficult decisions is a good idea. Upgrade your ship this way or that way? If you travel here to trade, you may miss out on an opportunity over there.