I noticed some NPC's (such as the high elf alchemist in Balmora and Ra'Virr) have stacks of ingredients that instantly restock when you barter with them again. When selling them the same item back, their own stack will double in size the next time you visit them, which allows you to easily abuse alchemy.

Sometimes happens with other items too. When you kill the NPC and loot the stack, it will respawn the next time you check the corpse.

Not quite sure if this is a known glitch. I have multiple mods that alter those NPC's, but can't seem to figure out which one is causing this.

No mods are causing this. Inventory Doubling is part of the base game.

It was how I could amass a lot of gold when playing on the Xbox. Tedious, but doable.

Now, it should not happen on a corpse, so that might be a mod related issue.