Selling macbook, best/fastest way to delete data?
I don't know if I can necessarily help you out with the "fastest" way, but I think I can help with "best".

Most of your data will indeed be in your account, but there's also the chance that other personal items may be scattered about your hard drive.  I would not personally sell a computer after just removing my own account, particularly if my account was not securely deleted, either by overwriting the unused disk space or by discarding the encryption key.  (Of course, there's always the chance a computer could be lost or stolen, too, so I use FileVault 2 to encrypt my system's disk in case I'm not given the opportunity to securely erase it.)

You say you don't have a startup disk, but since you have Lion, you should be able to use OS X Recovery:


Hold command-R on startup.
Use Disk Utility to erase the disk securely using Security Options on the Erase tab.
Start the installer to install a fresh copy of Lion.  (This may involve downloading the installer from the Internet… note this can be a very large download.  If you're feeling nice, you could take this opportunity to grab a copy of the installer for your MacBook's new owner.)