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When it comes to intent then there would only be two possible reasons that we can look where people do just really give out importance of being clean or non tainted coins either they do tend to hide something or just to make themselves to be clean.
About their prices i dont see for it to be premium or too much.I agree that there were some forensics or tracing teams but using basically a mixer would already messes up their entire
operation or motive.
I don't think you get that mixers don't cover up the entire trail. There is a small likelihood that forensic teams with sufficient technology are able to trace back the BTC to an entity, although unlikely. Or at least they are able to deduce that the coins came out of a mixer, and not out of a person's personal wallet.
While with virgin BTC there is no trail to cover up whatsoever. Obviously that has serious implications
From a legal standpoint, I don't think that selling or using these BTCs will be particularly illegal. After all, BTC is supposed to be fungible.