Buying traffic is often a risky business because most of the time you don't know where the traffic is coming from. Some of those companies offer traffic generated by pop-up windows or other tricks and as soon as the visitor sees he has been tricked into something he either clicks the back-button or closes the browser. This causes an entry in your access logfile, but no conversions. Even with the more legitimate companies the visitors are most of the time less targeted and less prone to convert than traffic via bookmarks, search engines or referrals from other sites in your niche.