I dunno how many you need but I can assure you that I an linked on all major sites in my sector, slowly growb=n since 1996 all within theme on non profit sites, the Association of my profession the major forum on our topic and so on.
BUT .. I have now a domain spam network linking to me, I didn't ask for it, but they now also mention SEO and bla on their sites.
How is Google gonna test if I bought these links, they can't , it's a ridicolous assumption to suggest they could unless I would have been so stupid to order please 3000 ibl's over gmail.
Google doesn't seem to get the refinement of pool and swimming pool, how can it possibly detect a complex human action.
But what they can do is # of ibl with same keyword and low PR > than x percentage ... RANK - 950 and accept the collateral.
Given that the 950 is near instant it isn't a gradient.
It's an if z > than x **** off to the end of the serps.
I find many pdf's at the EOS. They look completly harmless, have obviously no popups ads and so on. no boilerplate templates. Just plain text.