Okay, if it was pre-installed, then you are allowed to make a onetime transfer of the Lion license, so....
Make sure you back up the files you want to keep.
Use the internet recovery method to wipe the drive and reinstall the OS.
You will first get this window:
Use Disk Utility to wipe the drive since you are giving it to someone else. After it is wiped, choose reinstall Mac OS X. The aervers will identify your machine and should install the exact same version of Lion that it came with. Using Internet Recovery will tie the OS to the machine, so the new owner should be able to reinstall if necessary.