Daiyu's shop sells Healing Potions and Buff Potions from the start of the game. If Cure Poison could be bought too, that would make entering the Spider Cave for the first time a fetching mission instead of a Main Story mission.Cure Poison cannot be bought, I suggested adding that to Daiyu's shop a few posts back. I'm okay with Ward vs Infection not being sold, it's not exactly a common need.
The Spider Queen is one of the first Boss Figths. Poison immunity make killing it easier than killing a Wolf or a Bandit.
Before Daiyu's shop was added to the game we already could easily stock up on Bandages (100% heal for 40 coins) and buy decent Gear if you take the time to do a few missions around Weirdwood.
It would make sense to be able to buy Cure Poison after Zivah breaks her curse.