The chairs made by Clissett and Gardiner only look similar at first glance - there is quite a range of easy-to-spot differences. In an attempt to help, buyers as much as sellers, I've written a guide to distinguishing between the two. It's only in draft at the moment, but you can download it from the "Ladderback chairs" page, or try it out from here. Please let me know if there are any problems.
As an example, check out the chairs in the photograph. Even with a small blurry photograph like this one, it's easy to see that the side chair is not by Clissett. This is because the front feet are plain; Clissett's side chair had small ball feet. In addition, the back legs on both these chairs rake back sharply from the seat - typical of Gardiner and his followers, Neville and Lawrence Neal. Clissett's chairs exhibit a relatively smooth curve in the back legs. If you are able to get your hands on a ladderback chair like these, then there are more than ten features that will help you tell the difference.