Depending on your past selling record, i.e. how many items you sold through your Gameflip account and what types of items you are selling, your cash from sale proceeds could be held for a few hours to a few days before it can be made available for withdrawal. This mechanism is necessary to filter out bad sellers and nefarious activities from the marketplace.
As you establish a good selling record over time, the period that the cash from completed sales is held will be reduced and consequently, become instantly available for withdrawal.
To see how many days and hours are left for your sales proceeds to become available, go to Wallet > Activity select the month, and locate the sold items in which proceeds are being held.