I used a leader to help me trade for the full sparrow because my trade
had more than 15 items.
The girl with Sparrow agreed to use the CL as a go-between. She gave
the costume to the leader to "hold" until I finished trading my items to her.
Once I finished trading my items to the girl, the CL gave me the Sparrow
costume.
I'm sorry to say that the VMK home page has this to say about trades:
If you decide to trade an item and it isn't given back, we can't help you recover it, because you knowingly entered into the trade. Remember, you're responsible for your character's actions in the game no matter what.
That section was regarding people borrowing items, but this applies to any
trade you make in the game.