How do I wipe/reset my phone via iTunes in order to sell it if can't use the screen?

If you have logged into iCloud with the device, you have the option to wipe it and remove it from your iCloud account from the computer. See this support document for some further instructions. What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

If you did not have Find My iPhone activated with the iCloud account, then there isn't anything you can do if you cannot access the screen. I'm guessing that you have not synced that device to the computer and iTunes in the past? If you had, you should be able to access the device in iTunes, but to restore it, you have to enter the passcode for the device, if it had one on it. That would be impossible, as you probably already know, if the screen does not work.

You're welcome.