Can we use referral links - promote our sites

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Hi i'm still in Doubt if we can post a referral link or a link to a site. For example a member, lets cal him Mark is asking for a good book about dog training. YOu have noticed that a merchant on clicbank is promoting such a book where you get a piece of the sales price and you post it in the thread that Mark has started is it allowed. Or another example you have a website with an article about stock trading tips a members is asking for some tips about stock trading can i refer to this article on my website. Thank you in advance for your response.