i did not include a "legal" because legal is a construct based on "blind" legality checking. MEANING you REALLY DON'T know the history of the pokemon (who cought it, and whether or not they were physically at the event). These pokemon lay in this grey area of, maybe or maybe not. This is not an issue of maybe or maybe not.
In my example, I am taking the assumption that a kid went to Gamestop, and either by asking, or by an error in the date of the demo NDS lite, the kid acquired the celebi. The kid did NOT receive it from the wondercard on our site, the leaked rom, or another method (hypergts, etc).
I am tending toward what Guested says. If you can prove that it was acquired ingest (with good faith, and without illegal means), then it should be legit. Otherwise its illegal. There are some traders who use the same protocol when deciding if ANY event pokemon is legit.
Some traders will ONLY collect pokemon that they can PROVE originated from a person who was at the event.