If it were me, I'd sell the laptop sans hard drive. Or go out and buy a new hard drive, swap it for the current one, and sell it that way. Then there's absolutely no chance that they will ever be able to recover any info from your system.
If you don't want to do that, google DBAN. It's not a guarantee, but it's probably going to reduce the field of people who can recover personal data to recovery specialists and people with government sized budgets. It will also likely shorten the lifespan of the drive significantly.
I have a Dell laptop that I am selling and I wanted to delete all of my info on it and set it up so that when the next person starts it up, it will proceed with set up as a first time.