Prepare MBA for sale with Mountain Lion

2 options.

1.
wipe and reinstall ML then shut it down ready for the buyer to startup and enter all his details, updates and then download the apps he/she requires from the App Store
(once he/she registers that particular Air with Apple and logs into his/her App Store account, the available software should show for downloading).

2.
find out their details and do the above for them.

as you're getting a rMBP it'd be handy to have a method of fresh installing that as well should the need arise.
so you'll need a couple of of usb sticks - say a 1GB and a 16Gb

put recovery disk assistant one one - http://support.apple.com/kb/HT4848 - that's your bootable stick to access disk utility/

and ML on the other - http://blog.gete.net/lion-diskmaker-us/ - (it's good for ML also) - that's your 'net independent and much faster way of installing ML on your current Air and any future ML based Macs

You need to sell it with Lion installed, not ML, as when you purchased ML it was tied to your account, not the laptop.

Its the buyers choice to then upgrade or not

If you do decide to ship it with ML, make sure to give them a bootable USB (Google or Youtube can tell you how to do that) or a DVD with ML on it so the user can reinstall if he/she needs to.