When a sold listing is "Under Review", it means the buyer's payment is being reviewed. There's nothing you, as a seller, have to do.
Rest assured, our system and Support will take care of all verification. If the payment is approved, then the order will be released. If the payment is rejected, then Gameflip will automatically list your item back to the marketplace.
In extreme cases, "Under Review" statuses can take up to 3 days in extreme cases to be lifted, but the usual is few hours. Please allow enough time for the review to be concluded.