Unfortunately this is the way of the world at the moment with greedy people praying on the nieve. I have seen a lot of silver plated coins going through auctions at the moment with descriptions as the real deal and they are fetching three figure sums when these only cost a pittance. I personally have stopped buying silver coins from auctions unless I can handle the coin and examine it.
Just a side note, I always carry;
A neodymium magnet (15kg)
A jewellers loupe (20x magnification)
Pocket scales (accurate 0.001)
Vernier callipers (digital versions)
Numista.com has weights,dimensions and images of almost every coin made and this should help you avoid the fakes a little easier.