Avoid Buying & Selling Links which pass PageRank, make sure to add rel="nofollow" attribute if you are selling links. Read complete article: http://googlewebmastercentral.blogspot.in/2013/02/a-reminder-about-selling-links.html
Owning a Blog Network may not be a cost effective idea... Focus more on Good Guest Posting and Guest Blogging... This will help you get natural backlinks....
Link Exchange is also a No-No, especially if you link to site A and site A links to your site. There is however a proper and safe way to do a link exchange. When doing a Link Exchange, make sure it is a 3-way to avoid getting caught by G.
Oh ! I have some 3-way backlink . but also i have some paid link in the sidebar of my website. I have some website with PR(3-4) and i get 3-4 Dollar/Month for those link. so i think i have to avoid them. Thanks for the post .