Why Google has problem with websites selling links, If Google Guys thinks that this is not good for Search Engines than do not count such links but why to ban sites. Somebody can make a living on selling links on his website because Links are not just for SEO but also can be used for cheap advertising like adsense. Why Google want people to use only Adsense and nothing else. For the records my site didn't got banned selling links.
Firstly, the cynical answer: Google would rather advertisers spend their money on Google AdWords, so they use their influence over webmasters to discourage a potenitally competing medium of advertising. Secondly, the answer I believe is more accurate, is that it not easy for an algorithm to accurately determine whether a link is "sold" or not. Therefore, to reduce the number of sold links affecting their algorithm, they just ban sites that are caught selling links. You can do whatever you like with your site, including selling links. Google can do whatever they like with their search engine, including banning sites.
Because they want to protect their search engine so that those sites that are really good came up top. In other words, they want to ensure their search results are most relevant to what people are searching for. Link selling is to avoid manipulation of the search engine ranking, that's why they are very serious on this.
Well on my personal opinion, those who sell back links are direct competitor of them . They will loose more income from adwords if webmasters or site owner directly pay text link ads from the site owner. Unlike if you used adwords they have share income.