I personally wouldn't trust it. especially if you want to actually use the amiibo for the actual game and such. if you want them just to sort of have the for their looks then it would be fine but other than that, i'd steer clear from them.
Keep calm and play Animal Crossing New Leaf!
Also please leave me a rating! I'd be more than happy to do the same :)
Yeah I've seen them as well. They are custom made cards by the sellers for you to use in your game. The custom amiibo cards have a chip with the same Amiibo data that the actual Amiibo figures have so they will work in your game. Basically these amiibo cards allow you to have the characters and furniture that can only be unlocked through certain amiibo figures. For example, the inkling boy, inkling girl, or squid amiibo figures unlock a character named Inkwell in the game. Be aware these cards are NOT made by Nintendo so purchase at your own risk. Also make sure the seller has a lot of positive feedback if you decide to purchase or you risk getting scammed.