Myths & Facts About Links Relevance

1. I have to build/buy relevant links from webpages of relevant or reciprocal topics. Otherwise, the link will be defined as a bad one by Google "spider" when crawling my PR.

No, you don't. The SEs bots, e.g. Google "spiders", do not recognize  the concepts! They conclude that the link is relevant, if its anchor text is frequently used on YOUR webpage and not on the backlink page!
Moreover, there are plenty of blogs and sites which have infomation about different things which likes an author. That's why natural links may appear even on websites which aren't relevant to your site. Otherwise, the fact that you have links on irrelevant sites doesn't have a bad influence.

1. Your link anchor text must be relevant to your site content.

Yes, that's absolutely right. You site texts have to contain these inique and wanted words you usually use as anchor text of inbound link. Remember, Google's system is built upon the conclusion that inbound link anchor text can be trusted to accurately describe the contents of a webpage.
To choose the best keywords (for which you may want to buy links via LinlsManagement), we recommend your reading this article (How to Choose the Best Keywords for Your Website) and usin the Google Adwords Keyword Selector Tool.

2. The relevant links are more efficient in terms of SEO.

That's wrong. The relevance of the backlink page is not the factor  that influences on your Google ranking. And what does? The backlink page PR does, as well as the PR Weight that this very page passes to your site (learn more about the PR Weight).

2. It's next to impossible to define one's link relevance.

According to Wikipedia "... It [relevance] is elusive, because the meaning of relevance appears to be difficult or impossible to capture within conventional logical system" (click here to read the whole article "Relevance").

3. The relevant links are more profitable.

That's not true. The link building doesn'n aim at your site advertising through the backlinkpages. The link building is the best way for your site to get hugh ranking on search engines for your targeted keywords. In fact the search engines (like Google) are your best advertisers. That's why only high PR links are profitable.

3. The relevance has nothing to do with your site PR.

The SEO specialists' opinion is that the sites engaged in"excessive" reciprocical linking have no chances to be considered as valuable ones by Google. Why? Just because when hunting after the links relevance, those sites owners disregard the real valu of the backlink page - its Passed PR Weight.