Earlier quoted context omitted.
This has happened on a few occasions (all of them opioids laced with fentanyl by vendors). The community tends to name and shame such vendors a lot when it occurs, and markets will ban them; but the very anonymity linked to the process makes that easy to circumvent, unfortunately. In any case, the general advice to people ordering from vendors for which there hasn't been a longstanding reputation + published reagent…
During the SR era I was a viewer of the network but not a participant. I didnt realize the extent these vendors went to. I had a friend who did know one vendor in person and supposedly he was very very meticulous and calculated with his actions.
But then again you also have the (from the point of view of law enforcement) low hanging fruit: vendors that encrypt nothing, keep a full record of all their customers, tend to ship with very bad stealth, etc. those are the ones that tend to get arrested. Fortunately if you're a customer it tends to be easy to pinpoint those vendors - the lack of care and professionalism tends to display itself also in their profile or during simple communications.