I dont think this are issues with buying of links. If i could push a site ranking through having bought inbound links, then i would be able to push down the sites that are competiting with me. Instead it could be possible that Google has sharpen to links that are gotten through article submission, directory etc... because all these are not true "Votes"
Is your site built on dropped domain i mean some doing which is already used by some one dropped and re used for your blog If so you are in problem a while ago Aron made it clear You are in problems if you build on dropped domains(for getting age factor)
I got that problem too, I've lost 80% of my Organic traffic because of not listed at Page 1 anymore...