Those look a little pricy compared to the ones on eBay, but that will work and it's really not that much money either way. Since your phone is stock, just do a restore after they notify you that the unlock has gone through and that will apply it. You'll see this message in iTunes afterwards. It indicates that it's a permanent unlock opposed to a software or SIM unlock.
I posted the link for the purpose of VERIFYING and existing unlock, not purchasing an unlock. Privately unlocking was cheap and easy in 2012, but they changed the law in January 2013. It's not legal anymore so the vendors pretty much disappeared. The advertised unlocks you'll find online now are scams. You'll need to go through AT&T if you want to unlock your phone. They will unlock it once your contract is fulfilled.
There is no way to disable Find My iPhone for the previous owner. He should have restored it and disabled it himself. Since he failed to do this, it is likely that he sold you a stolen phone. Unlocking it will not allow you to bypass the Find My iPhone security. Try to return the phone to him and get your money back since you will not be able to activate it.