If someone were to allow the police to look through their bank records (what's done is done), and said person had used that bank account to buy bitcoin, then sent that bitcoin to person2 who then spent it on the DNM's and sent a (pretty small, less than 10 user doses) package to person1 and the package got caught in customs which is why the police came in the first place. Could person1 then say he bought the bitcoin for investment, or would the police trace it and figure out that's a lie?
Bonus question: if person1 also allowed the police to look through his phone and he and person2 had talked about drug experiences on social media (what's done is done), how likely is it that person2 will be getting a visit?
Person1 has not admitted to anything, but has consented to search of premises (nothing was found), phone and bank records.
Person2 has ordered multiple times. His house, phone and computer is clean, but he has bought bitcoins with his bank account. If he gets a visit should he deny all knowledge and consent to a search of house, phone and computer and ask for a warrant if they want to search his bank records or just deny all knowledge and ask for a warrant if they want to search anything? If he asks for a warrant, is it likely that they will actually get one?
I would guess this depends on whether or not its a DarkNet investigation - or they know about any involvement you may have had in the DarkNet. Hell, the cops looking at this shit may not even know what the DNM's are - that all depends on the case and agency etc.
Buying BTC is not illegal though.....(in most countries)....and neither is selling BTC.
What someone else did with BTC you sold them is completely on them, and blockchain analysis is still in its infancy as far as LE goes (building cases based on it and stuff).