Found this on the site FAQ section (hope it helps)
Transactions listed under the "Unfinished" tab will go away after 60 days.
- My trading partner and I accepted a trade, but now we want to cancel it. How do we cancel?
After a trade is accepted, both parties' collections are automatically updated to show that the cards agreed upon are currently tied into a transaction and are no longer available. In order to undo this and remove the transaction from the "In Progress" area, both parties should set the transaction to "Unfinished". After both parties have set the transaction to "Unfinished", each party should do the following:
- Click on the "Unfinished" tab in the Transaction Manager
- Click on the "Update" link next to the transaction
- Scroll down to the bottom and click on "Check Collection"
- Under the "Receiving" tab, use the "Choose an action..." dropdown menu to move the cards back into your wantlist
- Under the "Sending" tab, use the "Choose an action..." dropdown menu to move the cards back into your for sale / trade list
"They've done studies, you know. Sixty percent of the time it works every time." - Brian Fantana from the movie Anchorman