try read some of the other threads around here is basically due to the buying or selling of PR and lack of unique content, those are the general theories.
If there were a penalization - I believe would be caused by bad spelling and grammar. Sites with typos are getting banned right and left.
LOL, that was funny because the bid directories are new, and they are pr0 or low pr! In other words, when you don't have PR then you can't sell pr! anyways!
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The broader answer to your question is Google with fight/excluded any site that infringes upon their ability to make money from Ad Words. Selling links is a direct competitor to this business model for Google.
Thats it in a nutshell all it took was 2 sentences jackweb2020 You will have noticed that all/or most do not sell links, the services are for inclusion Reviews, which take time and money to accept, reject, delete or approve