Buying any type of goods or services on the black market is difficult because it's not legal.
Black markets are rampant because citizens want to buy things that are difficult to come by through legal channels.The underground market's many drawbacks include the risk of fraud, the possibility of violence, and being saddled with counterfeit goods or adulterated products, which is especially dangerous in the case of medications.
Because tax evasion or participation in a black market activity is illegal, participants attempt to hide their behavior from the government or regulatory authority. Cash is the preferred medium of exchange in illegal transactions since cash transactions are less-easily traced.