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NPCs require "memory" in order to open their secret shop.
From Talk:Secret Shops
Talking to an NPC too much causes them stress, but that has falsely lead people to believe that they should not talk to them at all during gifting.
The fact is that the NPC MUST have a high enough "memory" of you in order to open their shop, and the easiest way to do this is to talk to them with keywords.
https://www.reddit.com/r/Mabinogi/comments/6ptfnx/neles_secret_shop/
Link to logic/math source is in the comments there too.
I've edited the instructions on the page accordingly.