People who have a high PR website may sell a link (usually in the footer, on all pages) to websites in a similar niche to help them rank better. However, for some people its not blackhat enough. Spammers nowadays tend to use automated software like the following:
-xrumer - Developed by a few russian people and primarily used by russian pharmacies for fraud, this is the most powerful "spammer" too out there. You can literally spend $1k and get 1million backlinks within a matter of days and have your site in the #1 position for whatever keyword you're aiming for.
-scrapebox - Similar to xrumer, but while xrumer targets forums (vbulletin etc), scrapebox specializes in harvesting large lists of wordpress blogs (throught google + proxies/botnets) and then posts comments on them (usually all those generic comments you see on wordpress blogs such as "nice post" "good advice, bookmarked" etc).
I could literally go on for hours and hours on how people are exploiting search engines, I think that its prime time for Google to develop an AI for this rather then just relying on algorithms. There needs to be a better PageRank alternative that needs to be implemented.
>benefits
In the most basic terms, for spammers, more links = better search rankings = more traffic = more $$$
Sorry bro that ain't true anymore. Xrumer brutal blasts used to rule years ago, but I can assure you that just having a million of crappy profile/forum post links popping up at the same moment nowadays isn't going to help your site ranking as it used to be. It's just not as powerful as it was before the whole xrumer was translated and sold to non-russian speaking folks.
Xrumer is still being deployed by the "big dogs" but not to link your main, "money site" directly. More like to link to other pages with links pointing to your site (or to pages with links to other pages with links to other pages with link to other pages that eventually link to your money site). Never directly, too risky (even though it's debated how risky it is, but still why taking a chance to find it out).
Buying links today means (more like) purchasing services like seolinkvine.com or buildmyrank.com where you buy posts (about the kw you want to target and have on them a contextual link) on network of sites with an high home page Page Rank.
Nonetheless Google is catching up with these networks, recently deindexing a lot of their sites and making their users, who spent lots of money for those links, feel really, really, really bad.
Pros in tough industries had to adapt to Google's algo evolution: they now build their own networks, they put out decent content on them, they provide a good user experience, they tend not to over-optimize, they host them on different A-class IPs, they have their domains registered with different names/addresses/etc, they build multiple tiers of links to their backlinks with custom developed softwares (or Zennoposter) with private proxies and their 24/7 running servers and so on.
It's not a cheap process nor a quick one.
As for now and long-term rankings, spamming isn't just enough.
To be on the safe side you really need to follow Matt Cutt's evergreen advice: build a kick-ass site that even a human reviewer working for Google would love to see on the first spots.
Knowing how easy it was to manipulate google results (and to some extent, it still is) imo that's the right direction from the perspective of a everyday user who's searching for something that's not clicking on Adsense ads or buying something.
Just to be clear, there are 40 some factors that effect the value of a link, and that is just on a generic level, these factors carry different weight in different verticals, and based on other signals. http://wiep.net/link-value-factors/
I think we need to wait and see how google rolls out this new penalty to see how it plays out.
Should the human reviewer check every link by hand?
Think codecademy.com now first for "learn to code". I mean, is it best place to learn how to code? At the moment, I'd say it's not. Is it the third result "codeyear.com" (run by codecademy as well)? I'm sure there are better sites that could take that spot and provide more value to the user than that landing page pointing to codecademy (Nothing personal against the codecademy guys of course, big kudos for their venture so far and best wishes for the future).
The problem with today's algorithm is that it can't really tell which site is really good and which one is just popular, mainstream, consolidated, with a high PR. Think lifehacker.com second for "learn to code" (on my side of the world, on google.com and chrome incognito mode on).
Right now if every single newspaper of the world cover your site about how to build a homemade nuclear device and actually write something about the matter in the article (how it would be natural to be), use as anchor text "how to build an atomic bomb" and link to your site, your site will skyrocket to the top for that kw (and many others similar). Even if it's just a school project or a joke that made it to the news.
Where's the quality check?
Anyway I'm sure that smart people are currently working on these details and that a comment reply can't really do justice to the complexity of the issue.
Besides, if one really good one does exist, many/most SEO people would be out of a job; white or black hats.
By buying links (undetected) you can manipulate the quality and relevancy of your website in Google's eyes and cause it to rank better than it should.
I think algorithmically they can only discount links from bad neighborhoods, etc... but if they catch you, they will manually penalize you.
I think we are a ways away from social signals being a "big factor." While google does claim that they look at social signals, I think from a practical standpoint the only value they provide is raising a red flag when someone manipulates social signals like buying likes and plus ones. The same way google looks at a link profile of a site, they can catch a bad social profile that seems unnatural.
In reality, links are almost never seen as a negative. Which is really a shame, because it makes it harder for good-guy SEO's to legitimately build links when you can just buy 10,000 on Fiverr and not worry about it. And most black-hat SEOs buy links from many players, so even if some are discounted they won't usually still come out on top.
Google is getting better at it though. Recently they've been cracking down on blog networks. This is purely conjecture, but I do believe that links are on their way out as the primary signal.
That's the whole point of the Google backlinks - get as many links back to your site as you can, and you'll get good SEO.
And if you do this, and Google detects it, they nail you.
I'd never do it; it's just an interesting question.
And the implications of that make me shiver a little.
(Disclosure: I have no idea how much it would cost. But my hypothesis is that it would be the same as if the website owner had done it.)