A Silver Mario amiibo was sold on a Chinese website in blank packaging, and it was real. Something like this happened before when a Ness Amiibo was sold over 1 month before it came out in Europe. When used with Mario Party 10, it starts with a special silver token, which makes the Silver Mario amiibo silver when used in amiibo party. Otherwise, it is exactly the same as a normal Mario amiibo.
It is now available for auction on eBay. At the time of writing, it is over $1000.