Some knockoff plushies nowadays can look pretty convincing. It’s very difficult to tell unless you have both to compare with and even then it’s just minor things that would look off (e.g. the tags, or the way the material feels). On eBay at least, the seller’s location is a primary indicator; if they are based in China, Hong Kong, or… basically just any Asian country that’s not Japan itself, then I’d avoid the listing if you are concerned with authenticity. Also, if the price seems too good to be true, more likely than not, it is.