Stockpiling zero-day vulnerabilities

Especially when you're dealing with chemicals that people plan to put in their bodies, trust is crucial. This is indeed why people don't screw each other over. (With the Silk Road, which admittedly a very new invention, this is the only way to prevent foul play.)

Violence is even worse for business that unfair dealing, and everyone involved knows this. So maybe for large bulk deals between criminal enterprises, this is a factor, but certainly not for ordinary everyday drug deals.