I went unfriendly with a sleep wand to test what happens when I sleep NPCs (non-playable-characters). First test was Gomald. It isn't possible to enter the vaults when he is asleep. But you can wake NPCs up by hitting them once.
When asleep, vendors won't buy items. It's possible for someone else to pick up the item for free, if you are slow to notice.
A more dangerous version of this is at Tavelor's bar. An item placed on the table while Tavelor is asleep will be free for anyone to pick up. I showed this trick to the game's creator.
It remains to be seen if this will be edited, or simply considered part of the game's mechanics. In the meantime, be wary of any unfriendly players standing next to Tavelor with a sleep wand.
Below is a minor update recently added. A message is displayed whenever a player, or monster, picks up a recently dropped item. This helps against a different stealing technique - someone hidden by an invisibility potion who grabs an item being traded.