Haven't tested this on ALL trading partners, but with Vicar Humbletoes in Act 3, I accidentally drug an item from MY inventory into the trading slot of the Vicar, and it acted like the item was his. I tried to remove it and put it back into my inventory, but it wouldn't let me, and when I ESC'd out of the trade, it kept the item in the Vicars inventory. Had to save scum to get the item back.
After testing it further, it appears that it doesn't happen when you do it with an item that the original character you chose to talk to him with, but only when you change to a different character during the same trading interaction.
