Hello, Could you tell me if the shamisen is pre-owned or new? If it is new there is a probability. But if it is vintage, the ivory wouldn’t be completely white. It would be darker, maybe yellow-ish. It can be Synthetic Ivory 象牙調.
The price makes it a little bit doubtful, doesn’t it? Get a closer look on them and check for some kind of pattern.
The only problem with identifying by pattern is that I saw some synthetic ivory with patterns.
I would go for the price. If it is pre owned, the itomaki should be a little darker and if it’s new, the shamisen would cost much more. (Brand new ivory itomaki will cost you about $1000)
Hope I have helped 
But talk with someone with more knowledge than me, like Kyle for example.