About cheap traffic providers

There are many sites out there where it's possible to buy cheap traffic coming from a mixture of expired domains, ads, referral sites etc.

Please, anyone can give me some indications about how to distinguish those who are honest and really provide human visitors form those who are scammy and use bots/artificially generated hits?

I see that a couple of them have been approved as legit here on BHW. I have already bought from them and I am satisfied. But there are many others. Who would you recommend?