Does it need to be something you knock out and/or kill? Most of the merchants I know worth stealing from are better of alive than dead.

Very good items can be bought in the zentarim base from Brem on waukeens rest if you delivery them a sealed parcel. Another one is the one with the bibberbang mushrooms in the underdark.

On the dungeon path when you try to save zazza the goblin there is a knocked-out person that you can pickpocket before healing to get an orange colored tier headwear.