Some merchants show random empty slots in their inventory.
Examples are: Moira Camber at 28k 23k Camelot
Farley Daegal at 28k 8k Salisbury Plains (seems to be missing the Planing Tool and Smithing Hammer)
Penda Thatcher, loc 9262 52741 Avalon Isle, Empty slots on page 2 where he is selling cloaks and robes.
Annete Howell, loc 10691 52886 Avalon Isle, selling alchemy ingredients. Empty slot on page 3.
Thaddius Kelly, loc 10964 52668 Avalon Isle, Weapon Merchant with empty slots on page 1, 2 and 3.
Zinia Marsen, loc 34363 51704 Isle of Glass, Armor Merchant with empty slots on the page selling cloaks (same as Penda Thatcher) Meadow Williams, loc 38048 20899 Isle of Glass, First slot on page 4 is empty.
Abigail Dylan, loc 36686 20661 Isle of Glass, Page 2 has empty slots. (IDENTICAL to Moira Camber in Camelot. Same merchant template?)
Ariadne Thompson, loc 37476 22090 Isle of Glass, Page 2 CLoaks and robes, same empty spots as Zinia Marsen and Penda Thatcher)
Bridget Forge, loc 43820 26598 Aldland, Page 1 seems to be missing the Planing Tool and Smithing Hammer, Page 4 empty slot in slot #1. Page 1 missing items seem identical to Farley Daegal.
Isabel, Alchemy Merchant, loc 43946 26192 Aldland. Empty Slot page 1. (same empty slot as Annete Howell)
It appears the Alloy Weapon Merchants have the same empty slots, and when compared to Mithril Weapon Merchants they have fewer 1hand Slash, 1hand Thrust, 1hand Crush and 2Handed weapons.
I will add more merchants as I find them.