Look up photos of ArisakaType 99s, you will see the stock has a fairly reddish tinge to it. Not saying that this particular example is wrong, just that it looks odd from the photos.
It can be hard to tell sometimes if things aren't 100% correct, and not everyone is a expert (I know I am not, and most people aren't either). The rifle might be original, or the seller might think it is original, its hard to know without actually having a good look at it.