Can I still use the product key after wiping?
Q: Do I need to re-active Windows 10 after wiping?
A: No, you don't have to.
Q: Is my product key still valid?
I mean I want to sell my computer (Windows 10), but before the selling, I have some personal stuff want to erase, not sure if they are also on C Drive, so I made up my mind to erase the entire disk including Windows 10 and do a clean installation, but I'm worried about the license is valid or not by then.
A: Yes, you can still use that. In fact, you don't need any key for an activated Windows 10 reinstallation. I mean a clean install, what's more, according to Microsoft, the registration is store on the Activation Servers.
You can go the settings, system and security, Activation, Windows will show the info:
Windows is activated with a digital license linked to your Microsoft account.
And if you sign into your PC with a Microsoft account, the info goes like:
Windows 10 is activated with a digital license linked to your Microsoft account.
Which means you can change/upgrade your hard drive and don't need to buy a license but sign into your MS account to tell MS that you changed the disk.
Bear in mind to uncheck the box "enter your product key" when reinstalling.
This tutorial may help you when you had a hardware change.