As this is not a region vendor or a guild vendor, they have an infinite supply of their items. Nation's rank does affect which items he carries, as well as your nationality. He might not have some items if you are not Windurstian.
The stock for every store in every city depends entirely on the city's rank. If it's ranked 1st, it will have all these items. If it's in 3rd, it will have only a few.
Some NPCs you normally buy things from will tell you, "I would love to sell, but I can't import from ---- region." That means that if the city that vendor is in gets control of that region, you can buy goods from that NPC.