After a buyer has purchased your unwanted gift card, you have to wait 2 weeks before you receive payment for that gift card, also called the quarantine period.

This period is to allow the buyer to check the card and spend it. You are still liable for the balance on the gift card after this period and it is very important that you take care of the gift card codes and do not allow anybody else to see them. The buyer can still make a claim after this two week period, but at least they have had a clear period before you get paid.

see also Terms & Conditions clause 29