Well, just think about this. If you would have some good trafic , why would you sell it?If you are smart enough to bring good traffic, you are not so stupid to sell it but you would be converting it to cash by yourself My opinion is that 99% of traffic that is sold is just trash traffic, script generated traffic using proxies...etc So don't waste your money, even if is cheap
it is good way to start also sometimes you need a better traffic rank and you are just not there via natural search. buying cheap auto traffic than helps.
Do you have some personal bad experience with this? I understand with script trading you can end up trading your good traffic for a poor one. But when it's done by link exchange? Visitor clicks a link to your site, maybe with some content preview - he is interested - isn't that the best wuality traffic you can get?
I do not think that traffic trading is a good idea to promote your site (your business). Targeted traffic that generated using SEO techniques is much more effective.