Why Buy Through a Licensed Dispensary?
Black Market Cannabis
Seed to Consumption State documentation and reporting
Rigorous testing of product
Safe purchasing environment
Higher carry and storage limits
Accurate documentation of potency levels
In a recent study focusing on testing for contaminants found in cannabis purchased via the black market, studies found that upwards of 85% of the product tested would have failed state-regulated testing protocols. The grouping of illegally purchased cannabis showed positive test for not only mold and mildew, but pesticides know to cause physical harm if consumed by humans. 
Recent studies also found that flower purchased illicitly (black market) compared to being purchased legally had discrepancies in product packaging claims of THC content compared to what tests revealed. The range of accuracy in illicit product ranged from 43-73% accuracy, all in which had lower THC content than advertised. Legally purchased product had a range 0f 88-100% accuracy in THC potency, in comparison. 
There were similar results in the same study when testing edible potency. Edible potency in black market purchases showed accuracy ranges from 23-28% while legal edibles were from 84-100% accurate in potency claims. Most edibles purchased via black market contained fractional amounts of what was represented in the packaging. There was also uniformity variances from 27% to 30% in black market product, where the amount of variance in edible pieces purchased were as small as 1.7% to 5.7% when measuring gummy uniformity. 
There were also findings that all 6 of the black market samples contained multiple instances of microbial contaminants while all of the samples attained legally showed no signs of microbial contaminants. 
None of the illicitly purchased samples in any category would have passed state-regulated and mandatory testing for pesticides.