So i'm not sure this is a feature of hardcoe mode or not but absolutely none of the vendors are restocking their inventories, not in 3 days, not in a week, not in many in-game months. I've only noticed this recently due to my reclusive nature and lack of looting corpses except for absolute necessities.

I won't mind as much if it's a feature of hardcoe, but if it's a glitch then it is really kind of aggravating to not be able to sell my overwhelming amount of poison.