I recently purchased a new phone, and I’d like to sell my existing iPhone. I certainly don’t want the new buyer to see all my data, however. How can I reset my phone so all my data is gone?
You really don't want to give away your phone with your data on it that's for sure. You need to completely reset your phone to its factory settings before you do that.
However you probably want to back up your data or at least ensure that it's all stored in the cloud somewhere. Next you want to disable the "Find My iPhone" option in settings you. Yon't be able to reset your phone with that feature turned on.
If you have an Apple Watch paired to your phone this is the time to unpaired it as well. You do that in the watch app. Next you want to remove your phone from your iCloud account. In settings tap your username then scroll to the bottom and sign out. Finally go to settings general, reset and choose erase all content in settings. You'll have to confirm a couple times. Finally your phone will be reset to its default settings.
Now if you want to sell it we've had good luck on "Selling phones". It's a very honest service. They do a good job. They pay you on time and they pay you a reasonable amount. So if you want to sell your phone and don't want to deal with e-bay or Craigslist check out Gazelle.com.