The Law of Rune Attraction
I remember reading a thread about Elden Ring merchants saying something to the effect that they were foolish to be selling wares when they could just kill things.
There are a couple of things that make us Tarnished special in the game. One is that we see the guidance of grace, those sparkling spectral lights that indicate both the presence of a site of grace and the direction of travel toward our fate.
The other is that Tarnished attract runes. Not just from our own martial efforts, but if you stand on the periphery of any battle the runes flow to your character. Not to the winner of the fight, not to merchants or other NPCs, not even to the shard-bearers.
I don't know the lore behind it, but there's a law of attraction for runes that seems to apply to us and which would impede any murderous merchant from benefiting.