> There are a lot of 3rd parties in these market places that provide services such as chemical analysis of vendors products and make these results available. In fact, it goes as far as the very vendors selling the drugs pay to have their goods independently verified

Who verifies the "independence" of the "3rd parties" and/or the reliability or honesty of their results?

> As someone already mentioned their business relies heavily on their reputation so they tend to take care of it quite closely.

Reputation is important to any business. Hence the practice of dishonest/insincere/bought and paid for reviews or endorsements.