You don't find cocaine at a flea market because cocaine-sellers, not knowing that you are not a cop, do not show it to you. And even if they do show cocaine to pre-screened buyers, that guy could be an undercover cop or an informant. There is nothing inherent to the functioning of any market that makes it illegal.

Silk Road implemented some countermeasures aimed at preventing dragnet-style arrests and mass prosecutions based on casual scraping of the site and analysis of the blockchain, but it was still completely vulnerable to a buyer-informant attack. The real world metaphorical equivalent is that the flea market did not require shoppers to register their real names and addresses, and declare the serial numbers of any cash they might be using to make purchases. I do not know of any flea market that does this, or any that could even survive if it tried.

Was there anything inherent to Silk Road that prevented cops from using exactly the same buyer-informant model to prosecute contraband trafficking that they use for physical street-level buys? No.

And we are not the Stasi. We are not required to inform upon one another. Any duty to report is usually associated with exercising a privilege granted by the state. Actual misprision requires not only that the crime be a felony, the witness be aware that it is occurring, and that they take active steps to conceal it. Misprision itself is only a misdemeanor, and protection against self-incrimination still applies. If reporting the crime would implicate you in a crime, it is not a crime to not report the original crime.

You are suggesting that it is acceptable to seize a person's entire business and sell off its assets before they are duly convicted of their misdemeanor.

Thanks to the decreasing popularity of misprision among Common Law jurisdictions, it is usually only applied to actual agents of the state with some measure of public trust or responsibility. The flea market would likely only be liable for crimes committed by tenant-sellers if the conditions of their business license specified a duty to police and report and they did not.

Silk Road, as a business with no physical premises, could never be subject to such conditions, as there is no one with authority to license it to operate. The fact remains that even if there were tons of cocaine, Silk Road was not selling it, and had no particular interest in the details of the transaction beyond the amount of the commission.

It is very easy to say that Silk Road was a den of criminals, but we expect them to be sleazy and untrustworthy. The cops that we throw at them in the pursuit of law and order must themselves be lawful and orderly. When they steal from suspects, and establish by their attitude that everyone is a suspect, they present an existential threat to the security of private property, independent of any other crimes that may be occurring.

This auction is a slap in the face to anyone at all concerned about police powers.