Our engineering team has verified this and it is a necessary feature to the marketplace system.
In this case, there needs to be an activity by either the buyer or the seller within the 3 days, for the order to automatically complete after these 3 days is up. So by activity I mean, simply opening the sold transaction.
So by the information that you have provided above, the buyer did not even open their purchase and you only opened the order after the 3 days.
The rule above is applied to the Android and iOS Apps (App Clients), since they are only used for a low flow of transactions at a time. Now if you use Gameflip through the Web Client, which is usually used for a higher flow of transactions, just by opening the listings page the system triggers all sold transactions to enable the completion after 3 days (if the buyer does not complete the order before).
So I would suggest sending the buyer a message after selling the item like “Thanks for purchasing!” or simply just opening the order as soon as it is sold, if you are only going to use the Android App for your orders.
Sorry for any inconvenience and have a great day!