The best free tool to use to check backlinks to websites you might be interested in buying is the Yahoo! Site Explorer tool. When you buy a website, most likely the seller will highlight how many backlinks the site has but the seller usually won’t tell you if those backlinks are quality links or not. And regardless of what the seller does or doesn’t claim, you want to be able to verify the backlinks yourself. This video provides an overview of the Yahoo! Site Explorer tool and how to use it to verify a website’s backlink history.
Be sure to read the due diligence series to learn other things you should research before buying websites.