Pokemon FireRed – Mew Event Version
This is the first I've heard of this one, but the seller has had some nice Nintendo employee only stuff before.
Apparently,the cartridge was used at last year's Toys R Us Mew giveaway event. This is an official cartridge made by Nintendo that is filled to the brim with Mews. Lucky Pokemon trainers were able to visit Toys R Us last year and get a legitimate Mew - not a Action Replay or knock off.