http://tortradedolnv35gcqys5qsi5yqx6le5zbbmtkyvsgvhfakb5khffcid.onion/faq
If you have made an error in the trade terms and the buyer has not yet paid, just simply create a new trade. All unpaid trades are not stored will be removed from the system automatically. However, once a trade has been paid, it is now active and cannot be modified.