Buying traffic is always a lottery whoever you buy it from and in most cases all you will do is fuck up your pageview and "time on site" stats as the people arriving on your site often have no idea how or why they got there and leave just as quick, however I have in the past made small purchases before from both ero-advertising and juicyads, (see no refrral link means genuine endorsement) and can confirm that there were some good customers contained in their traffic and BOTH gave me customers who paid for the advertising many times over.
That said, even good traffic won't buy unless you can convert them and most traffic can spot an affiliate white label in 2 or 3 clicks or less.
I forgot to say....I have a very good tracking system and always use it.
