Google and Bing both seek to discount the influence of paid links in their organic search results. While a search engine can't know which links were earned vs. paid for from viewing the link itself, there are clues it uses to detect patterns that indicate foul play. Websites caught buying or selling followed links risk severe penalties that will severely drop their rankings. (By the way, exchanging goods or services for a link is also a form of payment and qualifies as buying links.
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