Having said all that, leaving positive feedback in return for payment is absolutely inappropriate.
As always, when extending one's hand toward another, one must exercise caution.
That's why both TMAN and I picked out words carefully as not to guarantee the absolute success of the product, merely its existence.
That being said, I believe that the number of positives that you grant should never exceed even half that of the number of negatives you have sent. Selectiveness of trust is very important.
Why is it people are so afraid of leaving a positive for the wrong person, but not afraid of spamming negative ratings? I have left a mere handful of negative ratings, because I almost exclusively rate people whom I have had a direct interaction with. I am of the opinion people should not be so desperate to find crimes and instead address the actual fraud around here. Of course tracking down and proving real fraud is a lot harder than playing Columbo with random hapless noobs...