I'm having trouble finding any open markets in the game. I have a list of Centrepoint Markets, but they're either shielded or have nothing in them. Does anyone know of any open markets that supply ships, weapons, equipment, droids, and vehicles?
The Commerce Centers that allow access to the NPC market, there should be one on each homeworld in the NPC city. Some may not have it as I didn't get to them all.
However, any of these places will access the NPC market which is linked at the bottom left of the sidebar, and usually doesn't have much of anything at all. Items are usually bought within minutes of being on there.
You would be better served looking to player run markets, either the CPM (http://market.centrepointstation.com/) or the commerce forum on here. However, these sales usually do not have delivery with them and you will have to arrange either to bring them to you, or you to them.
Unlike most games, you can't buy everything you need from NPCs. Try the resource ELlias gave, look around in the commerce forums, and look to see if factions are selling things you like. You'll have to buy things you want from players.