Trouble Getting My iPhone Carrier Unlocked

Sprint is not going to unlock the phone without the original owner's permission, because if they did unlock it and it turned out later that they had unlocked a stolen phone, the original owner could create big problems for them.

If the original owner refuses to cooperate, use the eBay policy where the seller misrepresented the item for sale and try to get a refund. (By the way, the only reason I can think of where the 'original owner' would refuse to provide unlock permission is that he is not the 'original owner' but rather someone who ended up with the phone illegally.)

If you cannot get a refund and the original owner will not provide unlock permission, you probably need to either decide to sign on with Sprint as your carrier, or enjoy your new expensive paperweight.

No matter what you think, Sprint makes the final decision, and they are generally not helpful when it comes to unlocking. As the phone was misrepresented you should be able to get a refund from eBay's buyer protection.