I've had to deal with fake coins on
ebay before, if the seller is reputable you should have no problem getting back your money, though you may be stuck with some shipping fees. I had proof (in the form of an ANACS bodybag) that the coin I had purchased was a fake. If these coins are obvious fakes (there not made of silver, ect) than providing proof is likely not nessesary. I would contact
ebay as soon as possible. By law the seller must refund you the purchase price at the very minimum.