What's actually changed for you though? Have you looked in the analytics to see what pages are no longer bringing traffic, what keywords are no longer bringing you traffic, that kind of thing?
Is it across all pages and all keywords or is it just a few high traffic keywords and pages?
Just because your niche has many link buyers doesn't mean that you're not getting penalised for it 
I don't think Google will have known or cared about those millions of pages, I assume none of them have shown up as a landing page for visitors so I would guess they were effectively invisible to Google.
Paid links (and indeed normal links), you may not be getting penalised for them, but if some of your highest value links are themselves being punished or in some way devalued you may be losing out there as well.
I asume no other significant change occured at this time?