Well, he got what he deserved, I have traded with him before so he should knew me, he clearly said that the Gengar was redis because the person who traded it to him told it was redis.
I've had similar problems before, someone traded me a Allie's Bagon (FairyWitch - serebii, Eeveelover - pokecheck) and then she came to my shop telling me it was her Pokémon and it should be non-redis, I imediately removed it from my shop. If I had the same thoughts of ShinyFrosslass I would tell her something like ''Its redis because the person who traded me told it was redis and end of story.''