Q: Find my iPhone

You should do what the article suggests How to erase your iOS device and then set it up as a new device or restore it from backups - Apple Support

If he has factory reset it and is setting it up as new it won't ask for your specific login. He could use any login he wanted and most companies like this one have their own apple ID. If it is asking for your specific login then he is trying to set up the phone to download everything you had previously back onto it. However, if he does use a different login, then you can just sign out of that and sign back into yours and download everything you had on previously back on, but without his assistance and without him knowing your apple ID. Hope this helps.

You need to remove the device from your iCloud account.

What to do before selling or giving away your iPhone, iPad, or iPod ...

tipsycat wrote:

 

Yesterday I received an email from the first seller saying the phone had been repaired but he can't do anything with it because its asking for my Apple ID and password

The iPhone is protected by Activation Lock, which is why it is requesting your credentials.

After you have removed the iPhone from your iCloud devices list (this being the other way to remove the lock, without needing to disclose your credentials to anyone), the new owner will need to restart the iPhone before beginning the setup process:

More information:

Turn off Find My iPhone Activation Lock - Apple Support