How to unlock IPhone 6 locked to sprint

Only the carrier it is locked to can unlock it.

You would need to contact Sprint and have them unlock it if you qualify for their unlocking policy.

Steps here

Unlocking your Sprint Device - Sprint.com

More details here:

How to unlock your iPhone for use with a different carrier - Apple Support

As a rule, Sprint will not unlock an iPhone unless it is active on their network. That means you would have to sign up for service with Sprint, activate the phone on their network, then request an unlock. You may need to maintain service with Sprint for at least 50 days before they will agree to unlock it. Sprint is historically the least cooperative US carrier when it comes to unlocking.

You have to contact sprint. I would have the seller contact them first. Apple doesn't unlock phones.

Yes. It's very advisable you do not buy a device locked to Sprint unless you plan to use it on Sprint.