locked to old owner's apple ID.

I'm sorry to have to tell you this, but you can't. That information is locked into Apple's servers so they're the only ones who can remove it unless you happen to be the original owner and can prove it.

The only thing you can do at this point is to contact the old owner and get them to remove the phone from their iCloud account. That will free it up for you to do a restore and use it with your Apple ID.

If they're unwilling or unable to do that, I'm afraid you're stuck with a useless brick. Suggest trying to get your money back if possible.