I've been buying links from a pr8 website for about a year now, and recently the site is no longer showing up in a google backlink check. Does that mean that there is a possibility this link isn't helping any more, like it got banned from passing PR for getting caught selling links?
No that doesn't mean so. Google doesn't always show all the backlinks to your website, but a sample of the links Google know. So, don't panic about it .. they don't have any sure-shot way to know if the link was paid or not, 90% chances are link is still helping you. you should be able to judge this from your ranking.
Are you using Google Webmaster Tools to check your backlinks. If not, you should do so, since it shows a lot more backlinks, with more details too, than the simple link: command.
Ive always wondered "how does Google know"? do you think it's staff note it down? or is it just an algorithm?
My rankings haven't fallen, and my site and the backlink site haven't had our PR's dropped to 0 or anything like that, so I guess it isn't anything to worry about.
It is a combination of human editing and algorithms flagging the URL. Text like Sponsored Links and Your Link Here will also set off a warning. You'd be surprised how quickly your site will drop PR once an authoritative link is removed. Also a good idea to make sure the directories you purchase links from are hosted on different servers and the content is related to your site. I recently came across a service in Australia where they would get you onto the frontpage of Google for any keyword or phrase. One of our clients told us they purchased this service. We checked on how it was done and discovered some 55+ backlinks to their website from directories all hosted on the same server. We then found the website of the company who offered the service and low and behold they were hosted on the same server, with the same IP address as their link farms..... AND even worse, their website actually said they sell this service in order to advance your ranks in Google's search listings! The moral of the story, if you are going to buy a link from a site make sure their site is not overselling their links.
Just buy links from trusted sources, they probably became know to google as link sellers, and this can be easy to spot from algorithms because of the pattern of links they have, eg they own 10 sites, and the same few websites are being voted for by their sites, also the linking strategy of the pr8 site might have been paid for