If the marketplace is committing a crime, then any money or property held or managed by the marketplace would be involved in the crime. While you might not have transacted directly with the marketplace, the bitcoins you received came from it.
One interesting bit of complexity comes from the fact that once a UTXO is spent, the bitcoins in it can no longer be distinguished from other bitcoins. That is why "taint" is always a percentage. This is similar to the problem of identifying tainted money in a bank account. However, while money is legally fungible (in the U.S.), bitcoins are not, and so they must be treated differently. I look forward to seeing how laws are applied and misapplied to this situation.
Not sure I would fully agree with that statement but respect your thoughts. At least in the United States there is two primary paths the government can take to forfeit funds. One being a criminal forfeiture meaning you would have had to been convicted of a crime and purchasing bitcoins on a darknet is not criminal under any statute. The second is a civil forfeiture which requires that the property (seized funds) is directly "traceable" to a "specified unlawful activity". There is a list of those activities in the statutes but again those activities are crimes.
Your analysis would then say if a bank was committing a crime your funds being held at that bank can now be forfeited.