Been wondering about this since launch, so maybe it's already been addressed, but why is there so much inconsistency regarding what I can and can not buy from NPC vendors?
My federation characters can't buy anything but Beam weapons/torpedos with energy credits (and ONLY Phasers/Photons).
I can buy Mk II to Mk XI Phaser Cannons and Turrets with refined dilithium, but can't buy anything less than Mk XI Plasma Turrets (which I might add are also mislabeled as "Rear Admiral, Lower Half").
Similarly, I can't buy Disrupter Beam Arrays, Dual Beam Banks or Turrets 'till Mk XI (Cannons however are available from Mk II onward).
On the KDF side, I also can't buy Disrupter Beam Arrays, Dual Beam Banks or Turrets with dilithium, only Cannons, and only up to Mk X.
Now to be fair, I have not done an exhaustive search of every known vendor in game. I only checked ESD, DS9 and the Qo'noS Shipyard. If there's a greater variety somewhere else I've overlooked, I apologize.