I have been searching through the web and I found this page that you may find useful if you are looking for a second hand kayak for fishing from – Second hand Kayaks
There are loads of pages to search through, but by doing a quick search in Devon and Cornwall I found 19 boats, sadly only 3 are SOT kayaks, but when you look further afield there are many more for you to choose from.
Of course there is ebay – Sit on top kayaks there is a large selection of new and second hand kayks there, with lots of offers like free paddles. You have to be prepared to travel to get a deal!
When you are look at a used Kayak check the hull for scratches and damage and if possible get the thing in the water. Scratches are bound to be found, but you dont want really deep ones which make the hull look thin and weak. Its also best to get one that has been kept inside, as sunlight detoriates the hull material.