I need help with mass effect 2?

Hey I am on my second playthrough of Mass effect 2 for xbox and First i am wondering why none of the stores on the citadel have anything for sale except for the shop where you can buy pets and stuff. other than that theres like 3-4 more stores that when i log onto their computer they have nothing for sale?? why is that? and also i heard you can buy extra star clusters to unlock new places to go?