Obviously, make sure everything in the computer works and that you’ve moved your data off of the hard drive. Once you’ve done that, use Data Sanitizer to completely erase the drive and make sure data recovery is impossible. The general standard is that it’s a good idea to erase and rewrite over a drive at least 3 times. The included sanitizing profiles feature rewrite standards that exceed those prescribed by the US Department of Defense and the US Navy. Even if you’re sure that the person you’re selling to isn’t going to do anything with your data, this is the only way to ensure that recovery is impossible.