I have a laptop with 1 TB hdd which I am selling. My laptop has contained many sensitive data over the years and I wanted to remove any chances of ever recovering that data. I have planned on using the command sfill of package secure-delete.
Note: I want to keep the OS intact and only clean out the free space
The problem is that it could take too much time doing the erase, so I wanted to know the best option to use with sfill or any other program which I can use to do the same.
No, you don't. Not only is that far more likely that you'll have missed something, it's bucketloads more manual work for you checking every nook and cranny.
Nuke the disk from orbit in a filesystem-agnostic way (per Rinwind's comment: 1, 2) from a Live CD. The wipe is going to take time. It has to write to every sector on the disk. A few extra GB for the installation won't make much of a difference. Set it off and leave it going overnight.
Then just do an OEM install. This will allow the new owner to select their username and computer name. An OEM install takes under 20 minutes from a USB stick.