Why is not Safe to Buy Backlinks. This article is mentioned in Wikipedia as a source.
The intent of writing this article is to help webmasters understand the risks involved and the low-value of using old growth methods.
Google now has much more developed blockers that ignore these paid or manipulated links and don't penalize them as harshly or as much.
Let's go through the reasons why you shouldn't buy links.
1. Paid backlinks may cause penalties
Search engines react when people buy backlinks. As a result, a result, they might remove your website from their index (demotion, thrown out of the main index, keyword-specific penalty, Google Penguin penalty) without any previous warning (unnatural link building methods). Buying backlinks are considered spam (black-hat) SEO technique. It can put your website out of business permanently.
2. Bought backlinks give less value than natural backlinks
Especially if the site linking back is not relevant to your topic/niche. For example a backlink from a travel website that points to a lawyer's website regardless if the anchor text is relevant.
3. I doubt if buying backlinks can improve your rankings
Does buying backlinks improve rankings rather than earning them naturally? Most of the time, they look unnatural (wrong placement, lack of niche relevancy, blog link farms). Earning backlinks can be quite hard, but also paying for links is not easy to swallow if they are ineffective.
4. Paid backlinks are time-sensitive
Either they disappear after a short while (usually the content is removed by the webmaster), or the site linking back goes out of business or penalized, i.e., cheap PBN link sellers dealing with high deindex rates. Backlink vendors or VA's that build links at public platforms are prone to spam (software blasts), and those links will not remain for long.
5. Paid backlinks reveal recognizable patterns
Publishers commonly use the footer or the sidebar (non-contextual) to link back, signaling an unnatural backlink. Other patterns include bulk external linking, abnormal external linking graph, not relevant topic, non-contextual backlinks, no real traffic, built for the only scope of selling links, etc., factors that augment the risk for receiving sites. Of course, there are cases where the trade is not recognizable, and both parties keep the secret and quantities low. In this case, if the page is relevant, then the backlink will count. Freelancers selling those backlink packages tend to stick to the same patterns for every new client.
6. Openly advertised backlinks
Prominent link brokers advertise their services, i.e., openly via email marketing, or have built a platform for trading backlinks. Such behaviors offer an easy way to track link schemes, and sooner or later, anti-spam teams expose entire networks. Some people argue with that assertion based on the fact that the web is vast, and Google has limited resources to monitor link trading. If that was the case, how come websites being penalized for unnatural backlinks?
7. Natural-looking backlinks are hard to find
It is hard to find paid backlinks with a natural pattern with niche relevancy and not coming from a link farm or a toxic website, thus favoring penalization. Sites selling backlinks offer no guarantee against Google penalties. The risk falls on the advertiser's shoulders. Even whey publishers provide reviews for their services does not change the fact that they breach search engine policies. Reviews can be leaked, too, or advertised publicly.
8. Self-penalizing behaviors
When the advertiser figures out that the previous set of backlinks was inefficient, he might try buying massive quantities of links, eventually getting busted.
9. Spending money on links limits growth
Link buyers are bound to link schemes missing opportunities to build up organic traffic, brand awareness, website authority, etc.
Gary Illyes (Google) at the last SMX Advanced Conference while on stage, he said that buying links from sites that sell guest blog posts and the like is the same thing as"literally throwing money out of the window."
And here is Gary himself responding via Twitter to someone that offered him some "good DA" sites.