I have MacBook Pro (late 2007) which I planning to sell. Currently it's running Lion. Is it possible to clean all data but leave Lion?
Yes, Just hold down cmd+R during startup and Lion Recovery springs into action; It lets you choose from common utilities: You can choose to erase your hard drive and reinstall a fresh copy of Lion.
P.S: In order to reinstall OS X Lion, you will need to be connected to an Ethernet or Wi-Fi network