Legitimate vendors are still legitimate. The article seems to focus on vendors that are umbrellas for collections of other vendors. The question may be does the umbrella (e.g., eBay, Amazon) have a responsibility for checking on the legitimacy of its sub-vendors? And do these sub-vendors know that they have fakes?
A parallel to this is that companies like Facebook and YouTube are not responsible (for the most part) for the veracity of what is posted on their sites. Of course this may be a free speech issue; is there freedom of commerce?