Sometimes you can just about get a better deal buy getting a new kayak, dealers have close out deals and such, its possible to get lucky and find what you are looking for used but a kayak is a very person-specific purchase. So my advice is if you want to get used at least paddle some new one and figure what you need. I might be able to help you if you tell me about your size and the fishing you plan to do.
I've got 2 Cobra Tourers and planned on selling one as soon as the site was up. I just put a center 10" hatch in it otherwise its stock and in excellent shape. We (kayakjoe and I) can set it you up with any accessories you want. I fish out of a Tourer and its a very versatile kayak. As Joey said thiugh used kayaks don't go for much less than new, especially in season and the Tourer is one of the more expensive SOT touring kayaks that makes an excellent fishing kayak.
Usually at this time of year the demo day deals are only 10% off the kayak and accessories. You can do much better. First determine what you want or the few kayaks that you want and then shop around. Keep in mind that accessories make a kayak a fishing kayak. By purchasing the 2 from the same shop you'll get your best deal. I know that I've had many people come to me with better hull prices but once they added in the accessories I had the better deal. Not many business have most of the accessories you'll want and need and you end up paying shipping to a lot of places sometimes for items that cost a little more than the shipping charges.
I have seen some used kayaks in the newsday newspaper and you can also check bargainnews.com...its going to be hard to find something fishable but you might get lucky. Its a good idea to get familiar with the different kayaks out there so you dont waste you time checking out something thats not good for you and your fishing