In the finished trades list, there should be some sort of icon next to trades that you haven't left feedback for, to remind you to leave feedback. Currently it's kinda easy to forget, since there's no indication in the tradelist for the person that's 2nd to leave feedback.

+1

The Red exclamation symbol should suffice. I'd be tempted to put it in the brackets of the Trades link on the left (like the chat message bubble).

Also a reminder email if a week has passed and no feedback has been left, since it could have been the user's last ongoing Deckbox trade and they may not be visiting Deckbox regularly.