I have this bag and it's awesome. You might want to ask the seller their reserve price -- it may be almost as much as a new one. In that case, you might want to buy one from a retailer such as Bloomingdale's or Nordstrom.
From what I can tell, it looks good. Also I'm not sure if I have ever seen this bag counterfeited anyway. If others have opinions, I'm sure they'll chime in.