It's trivially easy to create new accounts and maintain parallel alternate accounts on dark net marketplaces, even going so far as giving yourself fake reviews to pump up reputations.
This is such a common scam on dark net markets that it has a name, known as exit scamming. A vendor will build themselves up a good reputation (usually fake) and then scam everyone who transacts with them, cashing out on the reputation. Wash, rinse, repeat with new accounts.
You also run a risk when leaving a bad review. Even if you never buy from that vendor again, you could buy from one of their alternate accounts, and they'll send you fentanyl in your product as retaliation.
Beyond that though a hitman seems to assume some level of proximity to the target that authorities could use. Mailing drugs much less so.
And its partially a true story:
https://www.insidehook.com/article/military/real-life-jackal...
Plus you order your goods to the wrong address or use a fake name, and when the heat comes around you say "it wasn't me" and flush your drugs -- they're not going to chase you for months the way they might a murder case.
Obviously exceptions for the guys ordering kilos of coke every quarter, but as long as you put a little bit of effort into CYA no one is going to kick down your door for $80 worth of Molly.