Probably the same way they verify their drugs potency. 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 and issued with a "certificate" of how good their quality is which they display on the product listing.

These black markets or silk roads are not just a bunch of drug swinging deviants. A lot of the processes are very well thought out. As someone already mentioned their business relies heavily on their reputation so they tend to take care of it quite closely.