I never really consider it too deeply to be honest because ultimately it is a game, and game mechanics will trump most other things.

That said, in my head canon, other players we meet are just generic adventurers without any specific job. And I always saw tomestone vendors as not having a huge stockpile of items waiting to trade, but rather we come in with materials and specifications we want them to make. That is to say, sage weapons exist in Eulmore because we are specifically asking them to make it.