I might just be a noob, but I've spent the last several days scouring the surface of two major planets (Abregado-Rae, Coruscant) for a market in which to purchase stuff off the NPC market. It would seem logical to me that a planet as big as Coruscant would have a commerce center SOMEWHERE on its surface, but alas, I have yet to find it.
Is there a way to see what buildings are in a city without actually being directly above that city? Like an orbital scanner, or somewhere in the galactic map?
Commerce Centres have to be built by trading factions, and since they had little use for those before the market was released, there might not be that many around. You might want to try checking out those factions' websites if they have a list there.
You also need to actually have access to a commerce centre, and there's a good chance that factions will only open them to their own members. As such, you may want to contact your own faction or allied factions about getting access to a commerce centre or trading station somewhere. Otherwise, even if you happen to spot one on Coruscant, you may not be able to get inside.