> I want it to be clean, but I want the operating system to stay... I have windows 8.
That's simply not possible.
You can either delete the partitions and reinstall the OS, so that you do have a clean install but then the Windows 8 didn't exactly stay - it was deleted and replaced by another installation of Windows 8. This is basically what you want if you have the ability to install the OS again and have the product key to register it.
Or you can keep the OS and a lot of the stuff you've put on it. There will always be traces of your user account, even if you make a new administrative user and delete your old user and your old user folder in C:\Users. Even if you uninstall all your programs, they never truly uninstall. That is, the system is not in the same state as it was before you installed - it's merely not able to run that program any longer. But it sort of knows a little bit about the fact you installed it at one point.
If at all possible, you could sell it without the OS. Just make a bootable linux USB stick, then use it to delete all the partitions on your drive. This will remove all of your data, but the buyer will now be responsible for having a product key/Windows install method, assuming they want Windows.