i think i found at least 6 poisons in game so far, 4 of which have poisoned my blades.
alchemists/mages usually only restock mana, healing, endurance, wine, water and the 4 mana plants. All other potions, plants, fire arrows don't i believe.
smiths restock pickaxes, hammers, and rawsteel i believe. Crossbow bolts too usually.
hunters restock arrows and sometimes crossbow bolts. Sometimes endurance potions.
general vendors restock water, bottles, torchs, pickaxes, hammers, laddles
also i've never seen a vendor sell off any stock you trade them. i also have waited for sellers to restock and then i will round up their inventory to multiples of 5 by selling them my gear. If you do this I have found almost every vendor to not replenish there supply by adding more on top. Its useful to help track what is actually continually available.