While setting up a trade, I hit cancel on a trade by accident.
(Nothing agreed to, was just changing items/numbers on the proposal, when my mouse wandered to the corner.)

Most of the cards I wanted in the deal were listed on the cancelled trade.
Is there any way to undo the cancel, or do I need to start over from scratch?

Easiest to do would be to start over, we sadly do not support un-cancellation at this moment.