Custom NPC Vendor Inventories

[RubyKnight]

Is there a guide to this anywhere, or is it one of those things everybody has to discover for themselves?

[Adellie]

I think no one is replying because they're not sure what you're talking about.

Are you asking for a list of custom vendors and what they sell?
for example, the tool dealer in payon sells grapes - which are normally not sold by npcs. I think that is the only case.