This answer concerns the case when you are buying Bitcoins.
After you send in a trade request to the seller of Bitcoins, you have a time window to complete the payment before the trade is automatically canceled (the payment window varies depending on the payment method of the trade, normally 15 minutes by default). During this time you need to complete your payment and press the "Mark as paid" button. The seller is notified that you've made the payment and the Bitcoins will be held in escrow until the seller releases them for you when seeing the payment in his/her account.
If you paid for the purchase, but did not mark as paid before the payment time window expired, the trade will be cancelled automatically and seller's bitcoin will be unfrozen, please contact us immediately or email to us [email protected] with trade ref and kindly explain the situation and why you couldn't complete payment within the time window. We will try to Reopen the trade and frozen the Seller Bitcoin. In case the trade cannot be reopened (because seller withdrew his Bitcoin or sold to other person), we will try to reach him or add him to our blacklist to ask for help, but for those cases, it's out of our control, we will contact you if he come back and trade with us again.