its like a 10-15 % discount for an opened package i dont think is a scam, but you should try to get it tested before purchase. Pay attention to seller rep, and go with your guts, but its not super cheap imo.
So not a vast difference in price to make you think something is wrong, and that Ebay seller does have 100% positive feedback. All I'd question is the manufacturer's guarantee, a dead cert with Amazon but less certain with products sold on Ebay - - may be why it's cheaper perhaps? But I reckon it's genuine alright.