Are you good at following instructions? Then read on to wipe the drive. I'll give you the easy way and the more elaborate paranoid way. As far as OS installation, I'll leave that up to others who may chime in. If not I guess I'll try and help. Do you still have the OEM install disk though?
1) Platter data erasure the paranoid way.
A) Download Veracrypt from
here. You'll want the Windows executable (exe). Now install it to the laptop being sold.
B) Launch Veracrypt. At the top go to System | Encrypt system partition drive. Chose the HDD, select the whole damn thing, chose the hash (SHA512 will work), chose an encryption of a cascade of three ciphers. Like AES, Twofish, Serpent or something. Chose the option to save recovery media and not write to a disk like a USB drive. Don't chose wipe free space and let Veracrypt encrypt. It may take a very, very long time. Like over night.
C) Download Hiren's Boot CD
here or DBAN from the link I gave.
D) Download Ventoy from
here.
E) Write Ventoy to a
blank USB drive.
F) Copy/paste the ISO or image file of DBAN or Hiren's Boot CD to the USB drive that was formatted with Ventoy.
G) You now boot the USB drive. Read how to do that
here. (Scroll down to where it says "How do I boot from USB in the BIOS?." You want an F key that
ASKS what drive to boot from. You don't need to monkey in BIOS its self. Unless BIOS has no option to ask you which drive to boot from. In which case you need to set up boot priorities in BIOS so that the USB drive is first to pick from.
H) If successfully booted from USB the Ventoy menu appears and you can now start DBAN or Hiren's Boot CD. If using Hiren's Boot CD you want to find the program Eraser and use that to erase the entire drive.
1) Platter data erasure the easy? way.
Start from step C above. I'm done typing. LOL
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He/she is not going to know how to do that. Just guessing...