I have noticed so many people selling links here with little to no value based on domain age , domain authority , page authority or backlink profile . Here is a list of free tools I use to check link quality. majesticseo.com , Great for checking backlinks to a questionable site as well trust flow and citation flow and links indexed to the site. Sites with few backlinks pointing to them might not be doing your site any good even if it has a high PR . I also check how many domains are referring these links . 100s or 1000s of backlinks from the same referring domain might be a good indicator that the site is "weak" in terms of link juice. The domain cold have been repurchased and registered to a new owner with a good historic link history , but a weak new link profile. Google will update the link profile and PR and the link will pass little or no juice. seoquake.com Download this free tool bar. It will check PR, alexa rank, keywords in search results and one very important overlooked factor in backlinking to a site .... If the site is indexed in google ! if you backlink to a site that has no pages indexed in google stay away !! Good indicator the site has been deindexed and penalized .Also has a great tool that checks for follow and nofollow links . aherfs.com New links , % of types of links , lost links, site issues, and backlink profile . organic serp results is now gone but I replaced it with seobook rank checker. http://tools.seobook.com/firefox/rank-checker/ . free download and checks up to 200 pages deep for any keyword on google, bing and yahoo. Awesome free tool ! http://www.checkpagerank.net Checks for faked site page rank . http://moonsy.com/domain_age/ check domain age and links to other tools like who is info, domain ip lookup ,page authority and many more. http://www.opensiteexplorer.org great tool but unless you are going to pay for the upgrade you only get a few times to use a day.