Matt Cutts, the head of webspam at Google explains what will be the consequences of sites receiving links from sites that sell backlinks.

When a website is penalized for selling links, all websites which are linked to URLs on this site are penalized and they also get a penalty? This is an interesting question asked by a webmaster to Matt Cutts.
The head of the anti-spam Google responds in a new video, "Normally, when the engine finds a website that sells links, its PageRank will go down by 30, 40 or 50% to show that Google has less confidence in this site, and loses its ability to pass PageRank in the future."

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