If the site that is “giving/selling” the link is featured on a well known ad selling site like text-link-ads, then they might assume the worst. Also if the “selling” site is adding and removing a lot of links on a regular basis, then that might ring some alarm bells. I’ve also heard that they don’t immediately give full credit to links. There is some sort of waiting period before any value is given to the link. The reasoning behind it is that paid ads usually aren’t permanent and expire a few weeks/months after they appear.