Most internet sites receive about 75% of visitors from search engines like Google, Yahoo or MSN. Google is the worldwide market leader in the search engine market. Because of this fact it is very important for every webmaster in the world to archive good rankings in Google’s search engine result pages (SERPs).
The most efficient way to archive good ranking in search engine result pages is to collect a lot of inbound links from authority sites for a specific niche. For a lot of webmasters selling links was quite a profitable way to make some cash. The monthly price of a link is mainly based on the Page Rank of the site selling the link. The Page Rank is assigned by Google and is visible in the Google Toolbar. Recently Google penalized most link selling sites. Many of those sites lost some of their Page Rank. This is the reason why selling links is much more difficult nowadays or you will get much less money for links.
So for Google selling links is against their guidelines – link exchanges are against their webmaster guidelines as well. Though, Google suggests to collect many quality inbound links in order to archive better rankings in the search engine result pages.
May well known web directories lost most of their Page Rank as well. A lot of directories were penalized so most of them are useless for link campaigns.
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