Ya know, I was afraid of this too. Especially with all the materials coming in from crafting missions. But I tell ya, the only thing I don't sell out of within a few hours are the green crap gear. It's faster on the imp side, but rep side is pretty reliable as well. The fastest things are Advanced mods/barrels/etc and scavenged stuff.
It makes me think that there is a very good market for everything, but not as many sellers that are needed to keep the GTN robust and alive. It'll get better though. I'd guess it's something that takes a while to get everyone into.
As for the location, I personally didn't realize there was a cross faction GTN until this weekend when someone mentioned it in vent. And I had already quested through NS last week and never saw it. NS is just too out of the way to worry about unless it's something to make a profit, or using one account to transfer over to another. (I like to play my trooper while my husband like his warrior. Weird not playing together, although I do keep an agent near his level incase we do feel like it.)