What version of OS X did it come with andf what version are you now using?
If both Lion or if both Mountain Lion you use the built in Recovery HD, Command+r at startup then Disk Utility, to erase the Macintosh HD partition and then reinstall that OS.
If it came with Lion and you are now using Mt Lion you have to use the Online Internet Recovery system, Command+Option+r at startup then again Disk Utility, but you need to Re-Partition the Drive as One Partition to get rid of the Mt Lion Recovery HD poartition then select Reinstall Mac OS X which will reinstall Lion.
You can Not sell it with Mt Lion installed IF it came with Lion. If you upgrade from Lion to Mt Lion Mt Lion is Tied to YOUR Apple ID and can not be transfered.
Ok then restart the system and hold down the Command+r keys and that will boot the system to the recovery HD partition and then open Disk Utility select the Macintosh HD and then the Erase tab and click Erase button. You really don't need to do a secure erase as the Airs have a SSD which is not like a Spinning HDD and it almost impossible to retrieve any data from once erased in any form.
Once that is done, which will take seconds to a minute or 2, exit DU and select Reinstall Mac OS X. Point it to the Macintosh HD and Lion will be downloaded from the internet and then the install will start.
Once the install finish and the system boot for the first time just hold down the power button until the system turns off. When the buyer starts it up it will be like it is being turned on for the first time, going through all the initial setup screens.
There is only one caveat. That is the iLife Apps do not get re-installed with the OS. The buyer is going to have to Register the serial number under their name and Apple ID. Then they can download them from the Mac App Store for free.