Store rarity chances?

[Centaurishin]

What stores tend to contain what items? Is it completely random, or do certain sectors have a preference for certain items--such as Engi sectors liking drones, maybe the Zoltans have more lasers, Rockmen have a greater chance for missile systems? If so, what has a greater chance of what?

[Zuriki]

There are definitions for individual races store events, but I can't find anything that shows *what* they sell, I'll hazard a guess it's almost completely random except you won't get sold stuff like shields, if you already have that system installed.