Sorry if I offended you
I would put my head on a block and say that google does not remove 'some' pages when it penalises a site, it is all or nothing normally, and whatever it does is normally across the board. What google has probably done is prevent your site from passing PR, which in turn has stopped PR flow across your site, which in turn has resulted in the bulk of your site pages being dropped from the index for not having any degree of trust/link benefit pointing to them.
Google are getting really good at disguising how they handle sites selling paid links, and are quite happy to handle this manually as well in the following manner.
1. Someone (probably one of your competitors
) reports your site for selling links
2. Flag raised
3. Link map created for your site and cross referenced to other reported sites
4. Human visits site
5. Human applies Page rank flow block to site(s)
6. Pages drop like flies
7.? what next, what to do now?
In the old days it was simple. You lost your page rank, you lost all your pages in the index! Google get smart though, they are trying to undermine the entire 'selling pagerank' network thing.
You say it is easy to get PR if you are using your own sites, the danger of that is that Google are quite happy to bring down a network as they are to bring down a single site.
" don't link to bad neighbourhoods' etc
