do you know these people offering advice to buy traffic? When it comes to my hard earned cash I only trust people I know - ask yourself this if buying traffic was so good why isn't everyone doing it? I sit here 10 hours a day trying to get organic rankings on the SE's for traffic but if I could buy 10,000 hits for $100 that say converts at 1% thats a 100 sign ups

bingo I'm done, but in realty its not 1% in fact its probably not even .01%
For buying links the best place to try is the SE - do a search for the main term you want to rank for then send the site owners a email asking if they would be interested in selling you link - I would recommend paying for a few directly links to make sure your site has been crawled
I'm not saying these traffic sellers are cheats just the quality level of the traffic is so pure you will never see a sign up from it due to tricks they use to gain this traffic