The following are examples of link schemes which can negatively impact
a site's ranking in search results:
Buying or selling links that pass PageRank. This includes exchanging money for links, or posts that contain links...
Linking to web spammers or
unrelated sites with the intent to manipulate PageRank
ref. Official Google guide about Link schemes
But according to the same phrase you are NOT creating a link schemes if you sell articles in your blogs as long as:
- your article does not contain a link at all
- OR your article does contain a link that does not pass PageRank, thus the link uses
<a> tag (or an in intermediate page of your site that is blocked by your robots.txt)
Obviously you can ignore what they suggest and think: "well how does Google will ever know I'm selling blog articles?!"
IMHO (i don't work for Google) if you sell an article with links that pass PageRank just once in a while it'll be hard for Google to spot your blog as an affiliate of link schemes, but if you often sell articles with links, and even worst the links in the articles are not (or poorly) related to the main subjects of your blog, I would be careful.
Moreover new tools like the disavow links provided to Webmasters along with the spam report for paid links could easily change any webmaster into a squeaker.