The idea behind the legal requirement that links that are paid for (in reality, paid for in any way - even a recip link could qualify the link as "paid" from a legal standpoint) be marked as such is that failure to do so implies to the consumer that the linked resource is endorsed by the linker. For example, Google was threatened with legal action in several countries for not sufficiently distinguishing paid links on the SERPs, because this could cause the consumer (or visitor) to believe that those links were there because they were more relevant than the organic links.

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The best way to learn anything, is to question everything.