I'm preparing an old Macbook Pro for resale. I want to setup the Lion installation in a way so that the Welcome video plays when the buyer boots the MBP for the first time.
How can I accomplish this?
You can't — sadly, Lion has no welcome video!
(However, you can reset a machine to the first-boot state by simply booting from the recovery partition, erasing the drive, and reinstalling Lion. See this thread for more instructions.)