This is an easy question to answer: Don't buy links. That is, don't buy links if you care about ranking well in Google in the long-term. They can seriously mess you up.
In very simple terms, links are like votes to Google. More links can help a site "win" the Google election for a particular search. Problem is, people mistakenly assume they can buy the election by buying votes. But the system is much more complicated than that.
Google allows some links to carry more weight than others, if they come from trusted sources. It also considers the words in likely near the links, to understand what a page being linked to might be about. Lots of people link to Amazon with the word "books" in the link, so Google effectively considers that a vote for Amazon to rank well for the word "books." Since few link to Amazon using the word "cars," you don't see it ranking for that.
Google also discounts some links from sites it doesn't trust. They might carry no weight at all. Worse, in some cases, it might decide that a link should be used to penalize a site, if it believes the site knowingly bought a link to try and "rig the election."
Google can tell links are purchased in a variety of ways. For one, you know those out-of-the-blue emails that promise to sell you links that Google can't detect? They get sent to people at Google, as well. Google also has an entire team devoted to doing nothing but investigating web spam, which includes people buying links.
If those aren't enough resources, Google actually encourages people to report link selling schemes, especially as a means for those who are caught to help redeem themselves into Google's good graces.
Buying links is meant to be a shortcut to winning the Google election. But, ultimately, you don't even know if the links you're buying will work, and there's an excellent chance they could prove harmful to you in the long-run.
If you're seeking good links, the best way is the more time-consuming way. Search for the topics you want to be found for. Review the top sites that appear. Find sites that aren't competitive to you and visit them. Do you have content on your site that would be useful to the audience of the other site? If so, then email that site suggesting where it might be useful to link to you, and point to an exact page that would be most relevant.
People are also still finding success with tactics like creating infographics for other sites or guest blogging. But the crucial factor here is to be selective. Don't offer blog posts to sites that specialize in nothing but guest posts. Offer blog posts to sites that are well-read and have an important audience you'd like to reach.
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