You can erase your hard drive by putting in your Mac OS X Install DVD, and boot from there. Then when that boots up go to Disk Utility under Utilities just after you choose your language. Here is a support article from Apple on how to erase and reinstall the OS.
http://support.apple.com/kb/ht1820
It is illegal to leave software that requires user authentication on a computer you intend to sell. When you purchased that software you agreed with whatever software company that you wouldn't abuse the amount of user licenses purchased. By leaving it on your computer to "increase" the value you are abusing the software company TOS.
Plus if you copied those applications to your new computer, and if you sell your old one with the applications still authorized they can "call home" and that serial number can be flagged and all applications that are activated with that serial number can be deactivated.