deleting data from macbook air before selling

You cannot erase the partition that you run Disk Utility from. Depending on which OS originally came with your MBA, you'll either need to restart your computer with the USB thumb drive it shipped with (pre-Lion) or you'll need to restart with the Recovery Partition. If your computer shipped with Lion or Mt Lion, read the linked article to see how to boot into Recovery Mode. From there you can erase the drive and then restore the OS using Internet Recovery.