When a phone has been blacklisted, either from being reported stolen or from an account that has been deactivated due to non-payment, there is no unlock code that will get it re-activated. As a matter of fact there is very little the carrier will do to re-activate a blacklisted phone. It's just not in their best interest.
Now, if the phone has been reported stolen to the carrier, then it either is truly stolen, which having in your possession could carry criminal liability (possession of stolen goods), or it was lost by the rightful owner and deactivated. Or, it could have been reported stolen by the true owner to benefit from either insurance or selling it out from under a contract.
In any case, there are some serious shenanigans going on here and you need to resolve it with the seller and eBay quickly.