PUK code is the code you use to reset a SIM PIN if you have forgotten it. Nothing to do with the phone, except that the SIM must be in a phone to enter the code.
PAC is the code you use to request a phone number transfer between carriers (at least here - I don't know how that works in the States, which "boost" suggests might be where the question comes from).
For this there are 2 possible issues. If someone has paid off a contract (so they own the phone) and then sells it without unlocking from the original carrier then it's purely a technical problem of undoing the SIM lock. The original carrier can do that, but probably will only do it for the original owner (if they are still a customer). But in that case you may be able to unlock it unofficially.
The second problem is the one described above: the phone was sold before it was paid off and the owner then stopped paying the contract. In this case the contract has been broken and the carrier will blacklist the phone, and there is nothing you can do about that (in the UK where I am there is no way they'd remove the blacklisting for you).