I just bought a used kayak. Two sites I found useful were
www.paddling.net which has lots of new classifieds every day and
www.thewantad.com which is where I eventually bought one from. At The Want Ad you have to pay $1.75 online to view each new issue, they come out on Tuesdays I believe. But there are 20-30 kayak ads all in the New England area, or at least there were last week.