Congratulations! If your order is being processed it means someone purchased your item.
During this period, Gameflip verifies the authenticity of the buyer's payment. The order is released after process is complete. No buyers have access to the items (e.g. automatic delivery keys or skins) while an order is being processed.
In most of the cases, the processing period last no more than 5 minutes, but depending on the purchase circumstances (e.g. very high value items), it could take up to few hours.
Note that if an order is cancelled during the processing period, your listing will be put back on sale automatically.