“Hi there, I’ve received the device and was able to repair it, but have encountered a slight issue. In order for me to use this phone as my own, it first must be cleared from the Apple ID it was previously signed into via iCloud. This can be done remotely using the Apple ID and password this device is signed in to on your end to remove your account. Simply go to Settings on your current device, providing you are signed in using the same Apple ID as the device you sold me. Select this iPhone 6S from your devices list and tap “remove from account”. I have access to the email address associated with the Apple ID if that would help this process. In order to reset and activate the device for myself, this must happen first. I look forward to your reply and to solving this issue!”
Hopefully this is something like what you’re looking for? Feel free to use this directly; I can edit as you need too!!