well buying traffic is not legal according to search engines the visit's which come from direct reference is legal, whereas come from other sites or advertising is illegal..
Paid traffic is a grey area in SEO depending on where it's coming from. Link Farms and obvious paid links are frowned upon, however if you pay for a link that is in context and in content on another website, then Google will rarely be able to tell the difference, so it will be passed off as a natural link (which is good).
I suggest to work on whit-hat or natural seo for traffic, buying traffic is not good from the Google point of view.