google knows it all - not only about who and how selling links but:
1. how long vistors stay on Your pages and how deep they are going in order to get valuable information from browsing sites in Google search results
2. Google knows how much traffic You get
3. Google respects You if You provide good links for people, so take a look at the site before You link it from Your pages
4. and of course google bans some sites, my bidding directory now has PR2 instead of PR4 I got 3 years ago
Google knows Your site real value
about dumb question "how google knows"? Do You use gmail? sid down please and watch this movie
http://www.masterplanthemovie.com/