Even if it's totally legal (since few goods are supposed to be restricted), the fact it sold on darknet marketplaces which allows illegal goods is enough for government to take everything.
Maybe, maybe not. I don't remember anyone going through the courts to challenge the government. Maybe it didn't made sense with how broad powers US authorities seem to have when it comes to confiscating property.
Don't forget the costs of suing someone, it's simply too expensive for most people.