Please consider adding the ability to leave a reason for cancelled trades. The list might need to change over time, but here are some options I can see listing from day one:

• No response to offer
• Got a better offer
• Could not agree
• Disparate value (downtrade)
• Decided to keep card(s)
• Cards never shipped/received
• Duplicate trade
• Starting new trade with same partner
• Didn't read my bio/guidelines
• Partner's rating/feedback score
• Had problems shipping to partner's country before

I think that making this field mandatory might be a good idea, if only to help out newcomers that don't know why things didn't work out.
And there could be an additional option of "Other..." that lets you specify your own reason.

BTW--I added another bullet regarding the trading partner's score.