Its my first time selling my hardware so I have no idea about the best method for wiping data from the HDD and also make it unrecoverable. its not that critical but I really don't like that feeling. also one of my HDDs is too slow and I think its failing. And if there any thing to do for the other hardware parts before selling ?
just format the one that you want to sell, unless you are a spy, you have nothing to worry about. for the failing one, you could always drill a hole through it.
Before I sell or discard a drive I always write zeros to the entire drive. It can be done with diskpart by issuing the "clean all" command. Care is needed using that utility with multiple drives since it can be easy to wipe the wrong drive. Make sure the correct drive is selected. It does take a while so best to start it and come back to it after an hour or two. It's possible to boot into WinPE off USB to do it with a single drive in the system, but I normally just plug the drive into another computer to wipe it. Alternately a Linux boot USB can be used to issue the disk to disk copy command "dd if=/dev/zero of=/dev/<drive> bs=64k". The <drive> would be something like sda or sdb depending on the system. The "bs=64k" is optional but using a bigger block size speeds up the process a good amount. Again risky command if issued incorrectly with other drives on the system.
Your safest bet is to just sell it without your hard drive(s). If you think you can get a better price with a drive than get a cheap SSD for it and install Windows
If the data is so sensitive then don't sell it. Plus you aren't really going to earn too much from the sell. Even new Hard drives these days are so cheap and affordable. So just break it or if you are really emotionally attached to the hard drive then dig Giant hole in the basement and dump it there and never tell anyone about it.
When I sell my pc I remove my old hdd and change hdd with cheap ssd model 120 GB and then sell. Old disk I again use... When disk die then destroy complete see up post from @MS_User