Reformatting iMac and keeping OS X Lion

Hi guys, first post on here.

Basically i'm set to sell my iMac 21-inch which is running OS X Lion (downloaded from App Store), but before I sell it I need to wipe everything and and reformat it. Is there a way I can keep Lion as the OS and not have to revert back to Snow Leopard?

Cheers in advance!

If you have already bought Lion you can redownload it at any time from the app store

I would do a backup of the Lion installer when you download it so you can keep a backup installer on disk

How to Burn Your Own OS X Lion Install DVD or USB Drive

Lion is the first Mac OS that's tied to the user, not the computer. According to the terms and conditions, you are allowed to install Lion on all the computers you own. If you sell the computer, you no longer own it, so Lion should be removed. You won't have to buy it again, but the new owner would need to.