There has been a massive increase of Bootleg Sonic Adventure Plush's recently.
Here Big The Cat
There are more but I cant be bothered searching for them, they are all based on the Sonic Adventure series plush line, they are really pathetic.
I don't think that is an actual bootleg, considering that the Robotnik plush the seller has from the same line is exactly the same one I have at home. Toy Network did produce some shitty plushes, and they were horrid in quality.
The rest of the seller's merchandise seems to check out, so I doubt it is fake.