Simple. Hacks cannot mimic legitimates no matter how perfectly you made, especially events. It is human intention to make a 'perfect' Pokemon, but do they feel it is 'perfect' enough to keep and use for battle? No, they will give it away. Trust me in this. I am sure there are some trade shops are not as clean as you think.
Lord Fighting, just release the Celebi. If you get a legit one, you are no different from the people who hacked and offers it as they don't consider the receiver's lost.